Since the letter for these sounds are the same, how do you tell the difference?
You tell from the context, like knowing in English whether to pronounce Y as in "you" or as in "gypsy". But I can see it would be much harder without the vowel marks that Duo provides.
Which letters you mean? Or can you explain more? I'd like to help you.
In Arabic there is no difference between these two English sounds. "O" is also the same. All translate to the sound "و".
The same applies to the E, I, Y sounds = "ي" *_^