English problems

"Which" is self-explanatory;"which" is used for things and persons, singular or plural, subject or object. It has no possessive case.
In which:-----eg:-This is the M box in which money is put in.
by which:----eg:-The house by which was destroyed by fire was gone.
------------------eg:-It was a war by which many people were killed.
of which:-----eg:-On NY's Eve of which we celebrate;have gone.
------------------eg:-English, of which is my best subject, is out of teachers.
at which:-----eg:-I woke up at which time the day breaks in..
Actually which is used to present a choice from a limited number. The choice with which is usually made more explicit by "which of", eg:-Which of them is your best subject? "English".
eg:-Which of you boys can't do this gym?
eg:-Which will you have, Jade or Pearl ?

Nouns like boys,books,apples,sheep are "countable".
Nouns like bread,glass,sand,steam,air,water, drink are "uncountable",they stand for substances that cannot be counted;they can only be measured.Some of these words eg:-glass,paper,stone etc can be countable in one context and, with a different meaning, "uncountable" in another.

self-explanatory