"Sugar is not good."
Translation:Zucker ist nicht gut.
That is actually an incredibly hard question to answer. You could think of it like this: what is the implied question? That thing that you're questioning (i.e. the thing that in the question is replaced by an interrogative pronoun/adverb) is what you negate in the statement. That is definitely not a perfect answer though.
One thing I feel confident to say is that there is never any need to negate the copula ‘to be’ in German, negating the predicate does the job. Negating the subject would have a different nuance that could be captured by English as well: ‘kein Zucker ist gut’ = ‘no sugar is good’, ‘there is not any sugar that is good’.
When the verb carries meaning, then you have to look at what part of the sentence still holds true and which new piece of information makes it false: that is what's being negated. Of course this is more of a rule of thumb and consolidated conventional usages about the placing of ‘nicht’ take precedence over the specific semantics, especially when more than one part of the sentence could be negated to the same effect (in fact, in those cases German often allows more than one word order, for example: ‘I don't like pizza’ = ‘ich mag keine Pizza’ or ‘ich mag Pizza nicht’, although one of the two might be more appropriate in a given context).
Please see my answer above.
In short: because ‘Zucker ist gut nicht’ is wrong, i.e. conventional usage dictates negating the predicative adjective in copula constructions.
Sure, you could consider ‘ist gut’ as a whole predicate and thus put ‘nicht’ at the end negating it (like, in principle, following German logic), but that's not the path German grammar decided to take.
About your first question (which I forgot about while writing the first comment, sorry), the whole sentence is effectively negated if you negate part of it, but which part you negate matters in terms of focus and it can have different implications. Although the best way of thinking of what ‘nicht’ does at the end of sentence, is negate the verb or, rather, the predicate phrase. ‘Negation of the whole sentence’ was an oversimplification on my part.
The ch is the h in hue or human, a sort of "hy" sound. Once you can repeat that sound by itself (hy hy hy), just put it in the middle of the word knit and you've basically got it.
So in other words:
hue = /çu/ ≈ hyū
nicht = /nɪçt/ ≈ nihyt
You could also think of it like a light sh as in sheep or shop (This is not the same sound as before, but it can help get you to the light ch sound in German).