Since I can use ἔστι for “there is / there exist,” what do I use for “there was / there were.” Would I use some past tense form of ἔστι?
The only past tense of εἰμί is the imperfect, and yes, it may be used in that way. If you wish to use the aorist, Greek uses other verb forms to express the concept. You will learn this as you progress. And it looks like you are nicely progressing, so you will get there.
Ευχαριστώ!
The road is long.