These two words may seem like they have similar meanings, but they are not actually synonyms.

“To be” is one of the most common words in English.  It has two main functions.

– As a main verb, it is a copula—this means it kind of works as an equals sign (=):

John is a tall American (John=a tall American).
My house is on the corner (my house’s location=on the corner)
Two plus two is four (2+2=4)

– As an auxiliary verb, it marks the progressive tense and the passive voice:

Mary is sleeping right now.
The fire is burning out of control.

The man is wanted by the police.
Smoking is prohibited.

“To Stay”, on the other hand, means “to remain in one place” or “to temporarily reside”:

Whenever my wife takes me shopping, she tells me to stay in the menswear section so she can find me easily (I spend all my time in that one place).
We stayed at the Plaza Hotel in New York last August (we spent the night at the Plaza—it was luxurious!!!)

You’ll notice that “stay” doesn’t have anything to do with “to be”.  “Stay” is never an auxiliary verb, nor does it ever mean equals to—those jobs are handled by “to be”.  Even though “stay” has a similar appearance to a verb in Romance languages (Latin and Italian stare, Spanish estar), it has no actual relationship with it!

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