Although adjective clauses are very useful in both speaking
and writing, you should be careful with them. Here are two
more common errors that you should avoid:

1. Do not use who after prepositions.

When relative pronouns are directly after prepositions
(a common pattern in formal speaking and writing),
object forms are needed. Because who is a subject
form, it is noticeably incorrect in this situation:
whom should be used.

Examples

Bob is the person to *who I gave the report. (wrong)

Bob is the person to whom I gave the report. (right)

Is Julie the person by *who the report was written? (wrong)

Is Julie the person by whom the report was written? (right)

That letter begins "To *who it may concern." (wrong)

That letter begins "To whom it may concern." (right)

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In situations that are not formal, the relative pronoun is
often separated from the preposition. When this happens,
who is often used instead of whom:

Bob is the person who I gave the report to. (common)

Is Julie the person who the report was written by? (common)

Remember: Sentences like the above are not acceptable
in formal situations.

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2. Do not use prepositions with where or when.
Instead, use which.

When where and when are used in adjective clauses,
they are not used with prepositions. The relative
pronoun which may be used in this situation, however.

Examples

That's the room in *where I have class. (wrong)

That's the room where I have class. (right)

That's the room in which I have class. (right)

Is this the city in *where you were born? (wrong)

Is this the city where you were born? (right)

Is this the city in which you were born? (right)

Tomorrow is the day on *when he has his
job interview. (wrong)

Tomorrow is the day when he has his
job interview. (right)

Tom orrow is the day on which he has his
job interview. (right)

1990 is the year in *when they were married. (wrong)

1990 is the year when they were married. (right)

1990 is the year in which they were married. (right)