Pt.1 Both are asking for a specific answer. Both can be used in choosing between two or among 2+ different things but "which" is used for this case more, I reckon. It really depends on how you use it. For example, "Which is more appealing when encountering a difficult situation, to compromise or to perservere?" "What do you do upon encountering a difficult situation, do you compromise or perservere?" "Which one of those kids is your cousin?" -no "what" equivalent "What will I buy?"- asking for a specific answer "Which will I buy?"-asking for sorvific answer but with a more narrowed down choice as the question could extend into something like "which one will I buy of these two dresses/ of all these desserts/ of the three wallets?" Basically, which includes information whilst what may or may not include information.

Micah's answer for part 1 was great. One more rule: when used as determiners, "what"=which (or that)+ a noun I don't like what you cooked= I don't like the food which you cooked I don't understand what you said= I don't understand the words which you said. Part 2: These phrases are all used to satisfy an old grammar rule which stated that you shouldn't end sentences with prepositions. They make sentences sound much more formal. This agreement isn't worth the paper on which it's written. =This agreement isn't worth the paper it's written on.= This agreement is useless. Both of us will break the agreement as soon as we finish signing the paper.//You don't even own the land on which you built that house= You don't even own the land you built that house on. Do you like the accent in which I'm speaking?=Do you like the accent which I'm speaking in? Is that the country from which you came?=Is that the country you came from? Is that the country to which you're going?=Is that the country which you're going to? That which rises must eventually fall.= Everything that rises eventually falls.