Who or Whom?

The use of WHOM is considered a very formal word and is now generally replaced with WHO. WHO is the subjective form while WHOM is the objective form. 

Who (subjective: person doing the action) will mow the grass? 

Whom (objective: person receiving the action) did they pay to do the job? 

The use of WHOM in standard writing is limited to sentences where WHOM follows a proposition. 

The managers to whom you spoke are at the meeting.

Where there is no proposition WHOM can be deleted. 

The following sentence shows how it reads with whom included, then we have re-written the sentence to exclude whom. 

The workers whom the manager met are all going on strike.

The workers the manager met are all going on strike.

As you can see whom becomes unnecessary.