For this sentence "I know about what we need."
I don't understand why we use "what" we need.
What is the function of "what" ?
Is it a pronoun?
Can we use "that" instead of "what"?
Could you please explain it to me?
And could you give me some examples about using clause after preposition?
Yes, it's pronoun.
No, you can't use that, not in a word-for-word exchange, although one could say:
"I know about that which we need."
That sounds a bit stilted, though, in my opinion. I think you'd be better off saying:
"I know what we need."
(There's no need for about in that sentence).
