The subject in existential ("there is/are") constructions is the dummy pronoun "there," so you can't use subject-verb agreement because "there" is neither singular nor plural. You therefore have to use one of the other types of verb agreement: Proximity agreement - the verb agrees with the number of the closest noun, even though it's not the subject. This is generally what style guides recommend for formal writing. So since the noun closest to the verb in your example is singular "a cat," the singular verb "is" would be correct under proximity agreement. If the second noun were plural, you'd still use "is" because the noun closest to the verb is still singular: "There is a cat and two dogs in the yard." If "two dogs" came directly after the verb, you'd use the plural verb form: "There are two dogs and a cat in the yard." Notional agreement - the verb is conjugated according to the intended meaning. Since you're talking about two (or more) animals, you would use the plural verb form to reflect this: "There are a cat and a dog in the yard," "There are a cat and two dogs in the yard," etc. So both "is" and "are" are correct in your example (and in many existential constructions), though it might be advisable to use proximity agreement in formal writing. Answer from Boglin007 on reddit.com