"I'll go if he goes" should be accepted. As a translation studies graduate I've noticed a couple of things here at Duolingo, which I enjoy immensely by the way, namely that the concept of "equivalence" in the realm of translation is not always followed. "I'll go if he goes" is the correct and idiomatic rendering of the Hebrew phrase "אני הולך אם הוא הולך". If you want us to do more literal translations, which may not necessarily be the best translation, for the sake of learning and verb forms, I think you should, at the very least, have a disclaimer detailing this fact.
Are we supposed to hear any difference in pronounciation between עם (with) and אם (if)? I just redid both lessons in order to find that difference (if it exists), but I couldn't.
I think there was a difference in classical hebrew. 'im versus 3im. But both merged in modern hebrew. Maybe in rural areas you will find still the difference.
I think they are both said exactly the same! But different spellings of course...
Like many words in English
Jews who migrated to Israel from Arabic speaking areas generally maintain the ע sound because Arabic also has it. It's a pretty difficult sound to pronounce and most Israelis don't. I wouldn't worry about finding a difference in pronunciation.
How to differentiate between אם and עם? I am 100% sure that they're both pronounced the same.
(Peter Starbatty) 'with' is pronounced like 'am', and 'if' 'eem' (or 'im'), or something like that... :)
You don't really. You can say אני הולך ברגל if you want to specify walking which literally means I walk on foot.