Common errors in English: Usage of Relative pronoun as subject whom.

Relative Pronoun as Subject

Incorrect: I saw the man whom you said came here yesterday.

Correct : I saw the man who you said came here yesterday.

Note: In this sentence ‘who’ is the subject of the verb ‘came’. ‘You said’ is a parenthetic clause. However, in informal English ‘who’ can also be used as an object. 

For example: 

Who(m) do you want? 

This is the boy who(m) you asked to come.