r.1. Why we must use present simple at the end of this sentence? "he sat in black boredom and discomfort until, after an hour, sleep came."
we don't. "came" is the past tense of "come". The sentence ends as it begins, in the past tense.
would it be ok to use past perfect instead of past simple? Why/why not?
he had sat in black boredom and discomfort until, after an hour, sleep had come.
you can use the past perfect if it is the appropriate tense for your meaning.
i've found this confusing as there are words like "until" and "after an hour".
you might be confusing this with the present perfect, which isn't used with some adverbs of time. But neither of the above is in the present perfect.
2. "why we must..." or "why must we" ?
"why must we ...?" is correct in the above context. Or, "why do we have to ...?" this is appropriate for questions. If it's a statement, the order order is different: "why we must change the order, i don't understand", i.e. "i don't understand why we must change the order". This is not a question.
3. Why can't we use real condition in the 2nd sentence? They both look like real conditions to me..
1)if you put a piece of paper over comb and hum into it, the paper vibrates.
2)if you attach the diaphragm to a pen, the pen then will make marks on a piece of paper, according to the sounds that have been made.
the second sentence uses "make" not "makes" because you've used the auxiliary verb "will". It has nothing to do with contrary-to-fact statements.
if you hum onto the comb, the paper vibrates.
If you hum onto the comb, the paper will vibrate.
Student or Learner