The verb "to be" has a complex history.
There were four different verbs in Old English:
1. "Beon" meaning to exist
2. "eom" meaning to remain, (mostly used in the present tense) and
3. "wesan" (which tended to be used in the past tense) and meant dwell
4. "earun" (A northern dialect word meaning to exist)
There was also a prefix "es-" that also could be used to indicate that something existed.
These four expressions have become mixed and tangled into what we now think of as being a single irregular verb, with the following forms:
infinitive to be
present participle being
past participle been
first person present I am I was
second person present you are you were
third person present it is it was
plural we/you/they are we/you/they were
It is a mess: five different words mixed into one. But you have to choose the correct form, based on the grammar of the sentence. When asked to pick a form of "to be" it is asking for one of these