In formal styles, we usually put a preposition before the relative pronouns which and who:
Example: The speed (at which) a material is heated depends on its chemical composition.
Example: In Peters’ novel, (on which) the film is based, the main character is a teenager.
Example: An actor (with whom) Gels on had previously worked contacted him for the role.
Example: Her many friends, (among whom) I like to be considerate, encouraged her.
Note that after a preposition you can’t use ‘who’ instead of ‘who’, and you can’t use ‘that’ or ‘relative pronoun zero’:
Example: Is it correct for politicians to make important decisions without consulting the public (to whom) they are accountable? (not… the public to whom they are responsible).
Example: The valley (in which) the town is located is heavily polluted (not The valley in which the town is located… )
In informal English, we usually put the preposition later in the relative clause instead of at the beginning:
Example: The office (to which) Graham led the way was full of books.
Example: Jim’s football ability, (for which) he was noted, had been fostered by his parents.
Example: The playground was not used by those children (for whom) it was built (for).
In this case, we prefer ‘who’ instead of ‘who’. When defining relative clauses, we can also use ‘it’ or ‘relative pronoun zero’ instead of ‘who’ or ‘which’ (for example, the children (it) it was created for).
If the verb in the relative clause is a two- or three-word verb (for example, cross, fill, traverse, care, admire, endure, assume), we usually don’t put the preposition before the relative pronoun:
Example: Your essay is one of those (that/it) I will review tomorrow. (instead of… which I will pass through tomorrow.)
Example: She is one of the few people (that/that) I admire. (no… who I look up at.)
In formal written English, we often prefer to use ‘of which’ instead of ‘whose’ to talk about things:
Example: A large amount of oil was spilled, the effects of which (whose) are still being felt. (or… whose effects are still being felt).
Example: The end of the war, whose anniversary (of which) is on November 16, will be commemorated in cities throughout the country (or… whose anniversary is on…)
Note that we cannot use ‘of which’ instead of ‘whose’ in a non-defining relative clause:
Example: Dorothy was able to switch between German, Polish and Russian, (all of which) she spoke fluently (not… all of which she spoke…)
Sometimes we can use ‘that…of’ instead of ‘of which’. This is less formal than ‘of which’ and ‘whose’ and is used mainly in spoken English:
Example: The school (of which she is the principal) is closing (or The school of which she is the principal…)
‘Whose’ can come after a preposition in a relative clause. However, it is more natural to put the preposition at the end of the clause in less formal contexts and in spoken English:
Example: We were grateful to Mr. Marks, (in which) because we had traveled home. (or… in whose car we had traveled home).
Example: I now turn to Freud, (from whose) work the following quote is taken (or… from whose work the following quote is taken).