Can you explain and show me the usage of 'of which / of what' ?
"Which" is normal as the object of a preposition, and can be used to modify a noun. "What" marks a direct question, and is usually used by itself. Note that "of which" is often somewhat formal in tone (marked with * below when it's formal).
A Our conference is postponed because of the strike.
B Because of what?
A The airline strike. As the e-mail from my boss says, "Until the strike, the timing * of which is very unfortunate, ends, teleconferencing is to be used whenever possible."
Some more examples:
The things * of which we rarely speak.
What are the things * of which we rarely speak?
I gave him an account of all our activities. Of what?Of which activities? Here "of which" is not formal. The first question is more general (activities? dreams? e-mails?), the second question wants details of the activities.
Until the strike, the timing * of which is very unfortunate, ends,
Until the strike whose timing is very unfortunate, ends,
Yes, but informally. "Whose" properly belongs in the series who/whose/whom, and some people believe it must always refer to a person. You can say "whose timing" without any problem, but you should write "the timing of which".
The things of which we rarely speak:more general subjects that we rarely bring up in conversation. Dangerous politics, perhaps.
The things we rarely speak (better without which, and usually "say", not "speak"): more specific thoughts rarely spoken out loud. "You are a bastard son of a bitch", for example.