Most words of more than one syllable have an accent on the syllable that is stressed. For example: "μαμά" is pronounced..."maMA" and not "MAma". Only one of the last three syllables receives an accent. But the best way is to just learn the word with its accent just as you learn how to spell it. (There are one or two exceptions to the above but most important is that you use what Duo gives you to help learn
the DropDown hints...pass your cursor over a word and the correct answer will be shown. There may be a couple of choice...no problem use the top one.
REad the Tips & notes for each skill. At the start of the skill you'll see a light bulb image...click on that.
And read the comments for the sentences you'll learn a lot.
Of course you can ask us any questions you have. Oh, and have a look at the Greek Forum
HINTS ON HOW TO LEARN THE MODERN GREEK ALPHABET and HOW TO GET THE GREEK KEYBOARD https://forum.duolingo.com/comment/23430853
TIPS TO HELP YOU LEARN + HOW TO REPORT A PROBLEM https://forum.duolingo.com/comment/22424028
And check out the Greek Forum here with more links.