Modern standard German (in Germany) has rules about when to use each, which were modified in 1996: http://german.about.com/library/weekly/aa092898.htm
Can I raise the pronunciation of gehen by our patient voice person? Specifically, the first vowel sound. It sounds to me like a long 'e', whereas I thought a short 'e' extended was/is standard. High german. I've found discussions here http://forum.wordreference.com/threads/gehen-pronounciation.2486351/ and here (scroll down for the six lines) https://www.duolingo.com/comment/15599197 and an audio here https://youtu.be/NKBDJli_70c A Duden ref didn't play for me. Does anyone else hear this as an issue?
I had the same thoughts, but the modal verb lession discussion on DL (found on the website version, but I think not available on the Ipad or android application versions) includes this at the bottom:
"Müssen vs. dürfen A common problem for English speakers learning German is to use müssen right. Here's the problem:
Ich muss schlafen. (I must sleep.) Ich muss nicht schlafen. (I don't need to sleep.) Actually, the problem is in English. Let's look at the same example again, but use "have to" instead:
Ich muss schlafen. (I have to sleep.) Ich muss nicht schlafen. (I don't have to to sleep.) As you can see, if you think "have to" instead of "must", you'll be fine.
But how to say "must not"?
Ich darf nicht schlafen. (I must not sleep.) Ich darf schlafen. (I'm allowed to sleep.) As you can see, dürfen works pretty much like "may" in English.
Darf ich? (May I?) Nein, du darfst nicht. (No, you may not.) Oh, schade."
Both PONS and dict.cc give "müssen" as the German word for "must". I am currently trying to understand the word "müssen" as including both "must" (which seems to be a stronger/more intense concept) and "have to" (which seems to have a variability of intensity, depending on how strongly the person says the word). I think the work has nuances that do not directly translate to English, especially when used in the negative sense. ;-)