The verb τρώω (to eat) is irregular. Έφαγα, έφαγες, έφαγε, φάγαμε, φάγατε, έφαγαν are the simple past forms. Check the full conjugation table here https://en.wiktionary.org/wiki/%CF%84%CF%81%CF%8E%CF%89#Conjugation
You can say έτρωγες (staying with second person singular for this example), but that's the imperfect and doesn't mean quite the same: it refers to a continuous or habitual action in the past, so you might say "τι έτρωγες όταν ζούσες στη Γαλλία;", but not "*τι έτρωγες σήμερα για πρωινό;".
As always Greek speakers should feel free to correct me!
You are correct that "έτρωγα" is a way to put the verb "τρώω" in a past tense, but just a clarification: it is not translated as "ate" but as "were eating". When we learn English In Greece we call this tense Past Continuous. (I am not sure if this is the official name)
I'd agree with "I was eating" if you want a primary translation of έτρωγα.
However in English we quite commonly use the simple past to describe habitual past actions, where Greek would use the past continuous.
So "Όταν ζόυσα στη Γαλλία έτρωγα πολύ τυρί" could be translated as "When I was living in France I was eating a lot of cheese"; but if anything it's a bit more usual to use the simple past and say "When I lived in France I ate a lot of cheese".
It's the same in Greek, although we will most probably say: "Όταν ζούσα στη Γαλλία, έφαγα πολύ τυρί" to emphasize on the last part of the sentence. There are lots of ways to use tenses to emphasize on something you want to say, I'm not a linguist expert, I just thought that if a new learner read your fist post he would probably translate "έτρωγα" as "ate" and that won't be correct in most of the cases.
Here's a real-life example: https://www.in.gr/2018/12/06/plus/features/ti-etrogan-oi-arxaioi-ellines-poia-itan-ta-agapimena-tous-fagita/
<<Τι έτρωγαν οι αρχαίοι Ελληνες>>
i.e. "what the ancient Greeks ate [habitually in the past]"