Study materials
In these sentences the relative clause divides a group or does without a noun before it.
We interviewed forty people, most of whom had no experience.
Most of the forty people had no experience.
What surprised us most was the price.
The thing that surprised us most was the price.
Whoever finishes first can go home.
The person who finishes first can go home.
The article explains phrases such as most of whom and none of which, and the words what, whoever, whichever and whatever, which need no noun before them. Which about the whole previous idea is covered by the topic Non-defining Relative Clauses.
Most of whom and none of which
This phrase divides the group just named without starting a new sentence. The order is fixed: a quantity word, then of, then whom for people or which for things.
forty applicants, ten of whom were rejected
three reports, none of which was finished
two options, both of which looked risky
my cousins, all of whom live in Lviv
There is always a comma here, and never that or them. Of is required too, so most whom is a mistake. After the preposition of, people take whom, not who.
A superlative can replace the quantity word: ten rooms, the largest of which is full of maps. Whose works the same way: two students, one of whose parents is a doctor.
What, whoever and whichever
What means the thing that. It already contains both parts, so no noun stands before it. Whoever means anyone who, whichever any one of, and whatever anything that.
What we need is more time.
Whoever wrote this knew the system well.
Take whichever you prefer.
You can order whatever you like.
If there is already a noun, it is followed by that or which, not what: the thing that we need. All and everything are also followed by that: All that I want is sleep.
Common mistakes
The first mistake is a missing of.
forty people, most whom had no experience → forty people, most of whom had no experience
Of is required here.
The second mistake is a noun before what.
The thing what we need is time. → What we need is time.
What already means the thing that.
The third mistake is them instead of whom.
We hired five people, three of them speak French. → We hired five people, three of whom speak French.
Them turns two sentences into one wrong one.
The first and third mistakes show after the comma. If a quantity word stands there, it must be followed by of and whom or which. Them is possible only in a new sentence after a full stop. The second shows before what. If a noun already stands there, what must become that.