It should say in the tips and notes. Please read them for a fuller understanding of how the numbers works.
Shtaim is the number by itself and shtei is when it precedes a number of things.
How many cows do you have? כמה פרות יש לך?
I have two cows. יש לי שתי פרות.
You only get them when you're using the desktop version unfortunately (they aren't in the app). If you're using the desktop version, once you click on a lesson there's a lightbulb next to the 'begin lesson' button. They're definitely worth checking out for each lesson, even if you normally use the app
All the words with audio are here I'm learning Hebrew Duolingo vocab on Memrise! http://www.memrise.com/course/1031737/ (So recommend! I did the memrise Hebrew alphabet course too, I'll list it below)
The whole course tips and notes are here (and the site has one for each Duolingo language): ,organized by skill individually: http://duolingo.wikia.com/wiki/Hebrew
Or organized by skill in one pdf for the whole course: https://www.docdroid.net/JnfmyEV/tipsnotesbackup.pdf
Replace your username where mine is for information on your progress, etc: https://duome.eu/teribleteri/progress for progress and tips/notes set up by skill... etc
We don't have a separate word for "both", the difference between "two glasses" and "both glasses" in Hebrew is that the latter is definite: שתי כוסות / שתי הכוסות.
And then of course את is added because definite direct object: אני שותה את שתי הכוסות.
Now the construct in definite has two forms: the correct form, and the form we actually use, knowing it to be incorrect, to avoid social ridicule. The correct form is כוסות הבירה. The form that is used is almost always הכוסות בירה. So either say אני שותה את שתי כוסות הבירה and risk sounding very formal, or אני שותה את שתי הכוסות בירה.
Very in-depth answer indeed. However, I'd just add that there are still corners of Israel where the formal version would be happily accepted without any social ridicule and the incorrect one would meet the ridicule. Just come hang out with the "yekim" (Jews of German descent) in Haifa.
Could they add url links in the app, or search for "" in the discussions. I'd like to copy these answers but I cannot in the app and you can't easily search for the answers either. Even if I try to search for things I wrote and just use my username and words they barely show up.
To try something new in case this isn't an option:
(To see how searchable it really is).