know of

English as a Second Language English Vocabulary, Grammar and Idioms
1
  1. I know of what I am capable.
    Is this sentence grammatically correct?
    Can I say : “I know about what I am capable.” for it?
2

more probably,
I know what I am capable of.

3

“I know of what I am capable” is technically correct, but it sounds formal almost to the point of being strained. In everyday English you would say “I know what I am capable of”. Note that the “rule” about not ending a sentence with a preposition is largely discredited.

“I know about what I am capable” is not correct. You cannot be capable “about” something.