OF WHICH, THAT OF WHICH, OF WHOM
On WHICH 06 we covered the basics of 'to which' and 'from which'. Of course, as you learned on WHICH 03 and WHICH 04, the 'lequel' series isn't generally used for people, apart from a couple of exceptions - so you shouldn't be using 'auquel' or 'duquel' series words for people at all. How, then, do you say "to whom' or 'from whom'? Simply 'à qui' or 'de qui', generally.
'of whom' - and 'of which' - are a little more complex, however. This is because there's a specific (non-changing) word that can mean both/either of these - this word is 'dont', and it's used almost everywhere in preference to the 'duquel' series, in the context of 'of which', except after prepositions (them again). Note that 'dont' usually follows directly the noun to which it relates.
|He looked at the house, the garden of which was very small.
||Il regardait la maison, dont le jardin était très petit.|
|He looked at the house, at the end of which was a very small garden.
||Il regardait la maison, au bout |
de laquelle était un très petit jardin.
In English we tend to use 'whose' instead of 'of whom' (in the sense of 'belonging to'), so the position of 'dont' is more logical.
|I looked at the man whose car I had bought.
||Je regardais l'homme dont j'avais acheté la voiture.|
And finally ... you'll recall 'ce que' ('that which') that we met on WHICH 05. If you want to say 'that of which' (again a bit 'non-specific') you need to use 'ce dont'. "He didn't know what he needed' would be translated by "Il ne savait pas ce dont il avait besoin." (given that 'avoir besoin de' is the verb 'to need', 'to have need of'). Literally, you've just said 'He didn't know that of which he had need.", but English and French streamline wording in different ways.