Simple answer: because that's the way English works.

I am     -> I'm      ||  I have     -> I've
You are  -> You're   ||  You have   -> You've
He is    -> He's     ||  He has     -> He's
She is   -> She's    ||  She has    -> She's
It is    -> It's     ||  It has     -> It's
We are   -> We're    ||  We have    -> We've
They are -> They're  ||  They have  -> They've

There is a certain amount of ambiguitity here, but the precise meaning can usually be determined by context. You'll notice that the same ambiguity exists with he and it is/has.

Furthermore, it's (it is / it has) can also be confused with the possessive its (note no apostrophe)!

There are no contractions (that I'm aware of) for was (the past tense). She was is always She was, but She had can be contracted to She'd.

Answer from Mike Harris on Stack Exchange
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San Jose State University
sjsu.edu › writingcenter › docs › handouts › Contractions.pdf pdf
Contractions [pdf]
she’s  she is; she has · you’re  you are · shouldn’t  should not · you’ve  you have · that’s  that is; that has · One contraction that is not on the above list is “it’s.” It is useful to note that “it’s,” a contraction, is often confused with “its,” ...
Discussions

Are the contractions of "he is" and "he has" pronounced the same?
🌐 r/EnglishLearning
17
39
March 31, 2024
Can "she's" mean "she has" in written english?
Nonsense, but not every instance of has can be contracted to 's. I disagree with u/nihilcupio , you can't write "She's two apples"; people also don't say that, as far as I know. "Has" in the sentence "She has two apples" is the main verb, meaning "possess". But if "has" is just an auxiliary verb to indicate tense ("She has found her dream guy"), it's perfectly OK to contract: "She's found her dream guy" is OK in spoken and written English. Here's a wikipedia link, not the most authoritative, but carries some weight: Contracted auxiliaries The section also explain that 's for does is very informal: "I like that guy. What's he do?" And this is indeed a sentence I would not write down unless rendering speech. More on reddit.com
🌐 r/grammar
20
13
December 19, 2013
Do you use he's/she's/that's/there's as a contraction for/he was/she was/that was/there was?
No, all of those are contractions of "is," not "was." There is no contraction of "was" (except "wasn't").|@KaidenHsu No, "was" is never contracted this way. The mother is telling the stories in the present (or there's a transcription error in the written lyrics).|They aren’t the same meaning, ... More on hinative.com
🌐 hinative.com
3
March 25, 2020
She's [contraction 'she is' vs 'she was'] | WordReference Forums
What is the source please? Everything has a context. ... I understand vey well. Thank you Julian-san You have been helpful this time again! HIdenori ... Click to expand... Yes. I can't remember ever seeing "she's" as a contraction for "she was" but I can't say it's impossible. More on forum.wordreference.com
🌐 forum.wordreference.com
November 29, 2016
People also ask

What are contractions?
Contractions are two or more words that are combined and shortened to save time. Typically, you can identify a contraction by the apostrophe, as with isn’t or they’ve—but don’t confuse contractions with possessive nouns, which also use apostrophes.
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grammarly.com
grammarly.com › blog › grammar › contractions
What Are Contractions in Writing? Definition and Examples | Grammarly
When should you use contractions?
Contractions are considered informal, so they should not be used in formal writing like academic papers.
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grammarly.com
grammarly.com › blog › grammar › contractions
What Are Contractions in Writing? Definition and Examples | Grammarly
What are some examples of contractions?
Some of the most common contractions in English include can’t, it’s, they’ve, what’s, and would’ve.
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grammarly.com
grammarly.com › blog › grammar › contractions
What Are Contractions in Writing? Definition and Examples | Grammarly
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Quora
quora.com › Can-it-s-be-a-contraction-for-it-has
Can “it’s” be a contraction for “it has”? - Quora
Answer (1 of 96): Yes it can, however it is acting as an auxiliary verb in this form and needs to be followed by the main verb for that clause. I have added a list of all the contractions of English verbs which I can think of. Use them carefully because, like in the case above, they may not be th...
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Reddit
reddit.com › r/englishlearning › are the contractions of "he is" and "he has" pronounced the same?
r/EnglishLearning on Reddit: Are the contractions of "he is" and "he has" pronounced the same?
March 31, 2024 - The contraction of "he has" is pronounced hezzzzz. I'm from the NorthEast of the U.S. if that matters. ... OP ignore this comment. Both versions are pronounced the same: heez. Continue this thread · Can "she" be used as an impersonal generic pronoun too together with "he or she" and "they"?
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University of Nevada, Reno
unr.edu › university › writing & speaking center › writing & speaking resources › contractions
Contractions | University Writing & Speaking Center | University of Nevada, Reno
She’s = She is · How’s = How is · Contractions that take out the second letter of the second word: Note: Contractions that use the word “not” will replace the “o” with an apostrophe (‘). Shouldn’t = Should not · Wouldn’t = Would not · Couldn’t = Could not · Don’t = Do not · Isn’t = Is not · Wasn’t = Was not · Hasn’t = Has not ·
Find elsewhere
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Merriam-Webster
merriam-webster.com › dictionary › contraction
CONTRACTION Definition & Meaning - Merriam-Webster
4 days ago - The meaning of CONTRACTION is the action or process of contracting : the state of being contracted. How to use contraction in a sentence.
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Lawless English
lawlessenglish.com › home › contractions with the verb have
Contractions with the verb HAVE - Lawless English
April 18, 2014 - Has can never be contracted with its subject (he, she, or it) when it is the main verb in the sentence and is in the present tense.
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Cambridge Dictionary
dictionary.cambridge.org › us › dictionary › english › she-s
SHE'S | definition in the Cambridge English Dictionary
1 week ago - contraction of she is or she has: She’s (= She is) about to have her baby. I think she’s (= she has) already left the house. (Definition of she's from the Cambridge Academic Content Dictionary © Cambridge University Press) What is the pronunciation of she’s?
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Grammarly
grammarly.com › blog › grammar › contractions
What Are Contractions in Writing? Definition and Examples | Grammarly
August 10, 2022 - There’s another rule about contractions, again particular to American English, related to subject contractions in the present perfect tense (used for past actions that are related to or continue into the present). When the present perfect tense is used in the negative, it’s incorrect to use a subject contraction like she’s or I’ve. Instead you should use the subject followed by the contractions haven’t or hasn’t.
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Cambridge Dictionary
dictionary.cambridge.org › us › dictionary › english › he-s
HE'S | definition in the Cambridge English Dictionary
1 week ago - contraction of he is or he has: He’s (= He is) late. He’s (= He has) got $12 left. (Definition of he's from the Cambridge Academic Content Dictionary © Cambridge University Press) What is the pronunciation of he’s? in Chinese (Traditional) (he is的縮略形式), (he has的縮略形式)… ·
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Cambridge Dictionary
dictionary.cambridge.org › us › grammar › british-grammar › contractions
Contractions - English Grammar Today - Cambridge Dictionary
The following are the most common contractions. ... She is not is contracted to she isn’t or she’s not. I am not is only contracted to I’m not. Not: I’m n’t or I am n’t. They are not is contracted to they aren’t or they’re not. The isn’t / aren’t contractions are more common after nouns.
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Scribbr
scribbr.com › home › contractions (grammar) | definition & examples
Contractions (Grammar) | Definition & Examples
May 2, 2025 - Scribbr customers enjoy an ad-free experience! An apostrophe followed by an "s" may indicate possession (e.g., "Dave's house") or contraction (e.g., "Dave's here" = "Dave is here").
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Brainly
brainly.com › medicine › college › which of the following is a contraction? a. she's b. her's c. shes d. she has choose the correct possessive form of the word "dog." a. dogs b. dog's c. dogs' d. doges
[FREE] Which of the following is a contraction? A. She's B. Her's C. Shes D. She has Choose the correct - brainly.com
September 25, 2023 - In the given options, "a) She's" is a contraction, where "She's" stands for "She is" or "She has." It combines the pronoun "She" with the verb "is" or "has," demonstrating the use of an apostrophe to indicate the omitted letters.
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YouTube
youtube.com › english with lucy
How to pronounce the contractions in English - we'd | they'll | he'd | they're | it'd - YouTube
Learn how to pronounce and use contractions like I'M, WE'D, THEY'LL, WE'RE, THEY'RE, HE'D, IT'D. I will teach you the correct usage and pronunciation of thes...
Published   June 22, 2022
Views   603K
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LanguageTool
languagetool.org › home › contractions: what they are and how to form them
Contractions: What They Are and How To Form Them
July 29, 2025 - It has eight different forms, but only the present simple tense forms—is, am, and are—get contracted. Is is used for third-person singular subjects. It’s contracted by replacing the “i” with an apostrophe. She called to inform us that she’s going to be late.
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University of Sussex
sussex.ac.uk › informatics › punctuation › apostrophe › contractions
Contractions : The Apostrophe
But I advise you not to use the more colloquial contractions like she'd've in your formal writing: these things, while perfectly normal in speech, are a little too informal for careful writing. Such contractions represent the most useful job the apostrophe does for us, since, without it, we would have no way of expressing in writing the difference between she'll and shell, he'll and hell, can't and cant, I'll and ill, we're and were, she'd and shed, we'll and well, and perhaps a few others.
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Elephango
elephango.com › index.cfm › pg › k12learning › lcid › 11877 › Contractions:_Had_and_Have
Contractions: Had and Have Educational Resources K12 Learning, Grammar, English / Language Arts Lesson Plans, Activities, Experiments, Homeschool Help
When you combine a word with had, you add an apostrophe, then a "d" to the end of the word. Anytime you add the word "had" to the end of a word, you drop the h and the a and keep the d. Check out the list of contractions that contain the word "had" listed below.
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WordReference
forum.wordreference.com › english only › english only
She's [contraction 'she is' vs 'she was'] | WordReference Forums
November 29, 2016 - Even if the source was you. Sentences don't just float around in space. ... She's can also been a contraction of she has. She's been admitted to Standford University. However, I've never seen it used for she was.