Louis, "Which" is used as a relative pronoun, Basically that means that the word "which" is replacing a noun. This is done not to repeat the same word twice. For example: I will visit Brazil tomorrow, from which I will bring many presents. This is to avoid saying: I will visit Brazil tomorrow, from Brazil I will bring many presents. So you just have to pay attention to the preposition that would go with a noun and use "which" to replace the noun. Examples: Water has to be heated to a certain point, at which (at a point) it will start boiling. We have to enter the house, out of which (out of the house) those men came out. This situation has caused me a problem, which (this problem) I cannot resolve. I hope this helps.