What is the greek word for be?

There isn't one single Greek word for "to be," as the verb "to be" in Greek is conjugated differently depending on the tense and the context. Ancient Greek, in particular, has a more complex system than English. The most common forms you'll encounter are:

* εἰμί (eimi): This is the present tense form, meaning "I am." Other forms are conjugated from this root.

* ἦν (ēn): This is the third person singular imperfect, meaning "he/she/it was."

Therefore, there's no single equivalent. The appropriate word depends entirely on the tense and person.