Who and that are used when referring to persons. Select who when the individual person or the individuality of a group is meant and that when a class, species, or type is meant.
- She is the only one of my managers who can speak Japanese fluently.
- He is the kind of student that should take advanced basketweaving.
Which and that are used when referring to places, objects, and animals. Which is always used to introduce nonessential clauses, and that is ordinarily used to introduce essential clauses.
- Laura’s report on employee benefits, which I sent you last week, should be of some help. (Which introduces a nonessential clause – there is only one report.)
- The report that I sent you last week should be of some help. (That introduces an essential clause – there may have been other reports sent in other weeks.)
NOTE: Many writers now use either which or that to introduce an essential clause. Indeed, which is to be preferred to that (1) when there are two or more parallel essential clauses in the same sentence, (2) when that has already been used in the sentence, or (3) when the essential clause is introduced by an expression such as this…which, that…which, these…which, or those…which.
- Vivian is taking courses which will earn her a higher salary rating in her current job and which will qualify her for a number of higher-level jobs.
- That is a movie which you must not miss.
- We need to reinforce those ideas which were presented in earlier chapters.
Which, that, and who may be used to refer to organizations. When you are referring to the organization as a single entity (in other words, as it), then use which or that. However, when you are thinking of the organization in terms of the individuals who make up the organization (in other words, when you think of the organization as they), you may use who or that.
- Although you are free to choose your own doctor, medical care is also available through an HMO [a health maintenance organization] with which the company has a special contract. (Since the HMO is referred to as a single entity, use with which, not with whom.)
- Whenever we run short of computer supplies, the Brown and Weiner Company is the one that gives us the best service and the best prices.
- We really like doing business with the people at the Brown and Weiner Company. They are a customer-oriented group who give us the best service and the best prices. (That may also be used in this sentence in place of who.)
Source: The Gregg Reference Manual.