Hello there,
The verb "to eat" is τρώ(γ)ω and it's irregular. When θα precedes φάω, one of its forms, it is then the future simple tense in the indicative mood (θα φάω = Ι will eat), and when να or some other specific words precedes, it is then the simple past tense of the subjunctive mood (να φάω = (me) to eat) . Here is a table with the full conjugation:
http://moderngreekverbs.com/troo.htmlI hope this helps!
Stefania
Team GreekPod101.com