"He didn't go out last Friday."
"Mei you" here refers to an action in the past that did not occur. While "bu" should be used with the present tense. "Ta mei you qu" = he didn't go ; "Ta bu qu" = he doesn't go. However, i don't know if it is incorrect to use "bu" in this particular sentence instead of "mei you" since there is an element of time that already indicates the past (last Friday). In any case, "Mei you" just seems more appropriate to me.
Although they're both written as 'u', the vowel sounds of 出 and 去 are a bit different, and this is most obvious when they're next to each othet. 出 has the same vowel as 路 (lu, road) while 去 has the same vowel as 旅 (lü, travel). The ü sound is similar to German in that it's a 'oo' sound said while positioning the mouth for more of an 'ee' sound. Better explanations to be found elsewhere :)
The reason it isn't spelled 'qü' in pinyin is that q can only go with the ü, never u, so there's no ambiguity leaving off the umlaut.
Yes, the sound is quite different and it is supposed to. Q, x and j initials in pinyin are different from ch, sh and zh and so is the treatment of various finals.
To get a better idea of the sounds i recommend checking out an audio pinyin chart. The one on yoyo chinese used to be good and was free!
I think borth was talking about 去 changing pronunciation, not the difference between 去 and 出
My answer to that is maayybe kind of.
It's pretty common for second syllables to essentially have a neutral/softened tone (think unstressed syllable in english)
So like, while 去 is always pronounced "qu," depending on where it is in a word, it could be stessed a little differently and thus could sound like is has a different pronunciation.