A Thai verb has exactly one form. กิน is eat, ate, eaten, eating, will eat, would have eaten. There is nothing to conjugate, no irregular verbs, no participles, no agreement with the subject. This is not a simplification of a European system; it is a different system.
What English marks with tense (where an event sits relative to now), Thai mostly leaves to context, and marks aspect instead: whether the event is finished, ongoing, or not yet begun.
The three words that do the work
แล้ว after the verb says the event is complete. กำลัง before it says it is in progress. จะ before it says it has not happened yet.
None of these is a tense. แล้ว on its own does not mean 'past'. It means 'and that is done now', which is why it also turns up in sentences about the future.
- กินแล้วkin lɛ́ːw
have eaten / ate
complete: the commonest answer to 'have you eaten yet?'
- กำลังกินkam.laŋ kin
am eating
in progress
- จะกินtɕà kin
will eat / am going to eat
not yet
What to do with this
Stop translating tense. If you find yourself hunting for the Thai past tense, you are still building an English sentence and dressing it in Thai words.
In practice most sentences need no marker at all, because a time word has already settled it: say เมื่อวาน (yesterday) and everything after it is understood as past without further comment. Thai marks what is not already obvious, and nothing more.