In ancient Greek time wasn't as important as aspect. Thus the Greeks didn't make a difference between present and imperfect. They were the same.
The verb for "be" was εἶναι - "einai". Present/imperfect of this verb in 1st person singular was εἰμί - "eimi", which is etymologically the same as English (I) am. The Proto-Indoeuropean word was *h₁ésmi (I am, remain, exist), which is also the origin of Latin esse, es, est, estis, German ist, and war, English is, was and were, from the verb *wesanan. It's also the origin of Swedish vara, var, varit.