When can we use 'to be'?
Example: We Used To Be Friends.
what does the 'to be' mean?.
when can we use it?

Question

Answer

"used to be" is a phrase that means the same thing as "were in the past".

We used to be friends. (We were friends in the past, but we are not friends now).

Answer

"to be" (be) is an infinitive verb meaning to exist, occur or take place (depending on the context).  The conjugated present tense form of "be" is "(I) am, (he/she/it) is, (we/you) are.

It can also mean something that will be in the future; "She is my bride to be."