e.g.1. Who said that? -correct
2. Who did say that? -incorrect
Who is the subject of the first sentence above.
Another example : Who will make the correction? -will, here, is, in my opinion, the future morpheme for the future simple tense form of the verb make
The pronoun Who
The pronoun who usually refers only to persons. Unlike the other interrogative pronouns, who changes its form depending on the case, as shown in the following table.
Subjective Case --who Who did it?
Objective Case --whom Whom did you phone?
Possessive Case --whose Whose is that book?
When who is the subject of a verb, the subjective case must be used.
e.g. Who opened the door?
Who will help me?
It should be noted that when who is used with the verb to be, or with verbs in the Passive Voice, the subjective case must usually be used, since such verbs cannot take an object.
e.g. Who is it?
Who was the fastest runner?
Who will be there?
Who has been elected?
The first three examples above illustrate the use of who with the verb to be. The fourth example illustrates the use of who with a verb in the Passive Voice.
In formal English, when the pronoun who is the object of a verb or the object of a preposition, the objective form whom must be used.
e.g. Whom did you see downtown?
To whom did you send the invitations?
In the first example, whom is the object of the verb see. In the second example, whom is the object of the preposition to.
In informal English, the form who is often used for the objective as well as for the subjective case. For instance, in informal English, the preceding examples might be expressed Who did you see downtown? and Who did you send the invitations to? However, this use of who is considered to be grammatically incorrect in formal English.
source :English Grammar
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