How do I know when it's spelled
ι and when it's spelled
η and when it's spelled
For that matter, how do I know when it's
ο and when it's
You don't! Learn every word with its spelling, there are some rules but they are not enough to explain why words are spelled the way they are. Something you will see for example in the next lessons is that active voice verbs in the first person always end in -ω, like κάνω-to do, θέλω-to want. No exceptions there, but in general the rules are for specific cases only.
It does seem that the words they've taught us so far tend to be spelled -η if they end in /i/. I just need to remember this.
Yes, if a noun ends in /i/ in nominative it is always -η, 'cause it's a case ending