I was listening to my wife speak with her family and noticed that even though she was using a feminine noun(let’s say ‘cama’), she still said the number 4 as “cuatrO.” I asked her about this and she couldn’t explain why, so I’d like some insight if anyone has any. The examples I used with my wife to ask questions were:

Me — If i said I had one bed, how would I say it?

Her — “Una cama”

Me — If I had four beds I’d say?

Her — “Cuatro camas”

So my though now was that maybe the grammar just didn’t allow for differentiating being saying “One bed” and “A bed,” so I changed my question

Me — If you had four beds and I asked you how many beds you had, you’d say?

Her —“Cuatro”

Me — And if you had one bed and I asked you how many you had?

Her —“Una”

So why does the number one switch gender, but the other numbers don’t? Doesn’t this “break the rules?”

I'm obviously a beginner. Would 'My birthday is on the 30th of January' be 'Mi cumpleanos es la treinta de enero'? Or 'el treinta'? My dictionary doesn't give a gender for numbers.