For English "S", it's easy: use ς at the ends of words and σ everywhere else (beginning or middle).
For English "O", there's no way to know whether ο or ω is appropriate in Greek, unless you know whether the word used to be pronounced with a long or a short vowel 2500 years ago -- it's like English "ee" versus "ea", which used to be pronounced differently and were therefore written differently but are now pronounced the same and you just have to learn that it's "treat" but "tweet" etc.
Same story with η ι υ οι ει.
One small ray of hope: the above is true for word roots, but endings are more predictable. For example, the noun ending "-os" is -ος while the adverb ending "-os" is -ως; the passive ending "-ete" is -εται while the "you all" ending in the indicative active is -ετε; and so on.
ο, η, το are the Greek definite article, corresponding to English "the".
Roughly speaking, the definite article in both languages is used when you are speaking about something that is already known to the listener - often because you have spoken about it before.
The indefinite article ("a, an" in English) is used (roughly speaking) when you are speaking about something new.
For example: "Yesterday, I saw a man. The man was selling balloons."
In the first sentence, he is "a man" because he is new information; in the second, he is "the man" because the listener already knows which man you are talking about.
The use of definite and indefinite articles is a bit more complicated than that, and not quite the same in English as in Greek.
Quick question here: All these verb endings (-ω, -εις , -ει, etc) are used for all of the verbs in Greek (and then auxiliaries are used, like in English)? Or are they only like that in the present tense? Or are there irregular verbs in the present that wouldn't follow that pattern? You know, Greek has been very difficult for me, but I think I'm starting to get the hangs of some things. Not sure about verbs, though.
And I have another question: "ένα" here is in the accusative, right? Is "μήλο" a masculine or neuter word?
Sorry for the questions. I'm really into this lol
First of all don't try to equate what one language does with another language does. You will only get confused.
Yes, Greek uses endings which show the person (-ω, -εις, -ει, etc) (I, you, he.....etc) Yes, these are the present tense.
"used for all of the verbs in Greek"
No, of course, not...the verb changes according to the tense. As we say. "I go to school." or "I went to school." etc
"(and then auxiliaries are used, like in English)"
Sometimes yes, sometimes no. So, learn the Greek as Greek, not compared to English. It will make learning more productive and easier.
"ένα" is used in various ways see here: http://www.neurolingo.gr/online_tools/lexiscope.htm?term=%20%20%CE%AD%CE%BD%CE%B1
Ok, now stop worrying about all that.
The way you will learn on Duo is by doing all the exercises and getting used to the language. Later you can look at the grammar. Follow these instructions and you'll do well...and it will be a lot easier.
TIPS TO MAKE LEARNING EASIER + HOW TO REPORT A PROBLEM
It really works.