When vs. Where

According to traditional rules of usage, when is supposed to be used in reference only to words relating to time, and where is supposed to be used only in reference to places:

The spot where I feel most content is Sunny Cove. (A spot is a place.)

The Qin Dynasty is the period when China was first united. (A period is a time.)

However, these words are often misused in such constructions:

The candidate gave a speech where he criticized the incumbent governor.

He taught us a method where we could improve our shooting.

This is a case when caution is required.

Obviously, speeches and methods aren’t places, and a case is not a time, so these sentences should be revised. (Note that many good writers choose to bend these rules for the sake of simplicity and a natural-sounding style. Some such “incorrect” uses of when and where sound more natural than others. As a writer, you will develop your own sense of what sounds good by exploring different options.)

One good way to avoid these errors is to use the pronoun that or which instead, but you often need to use an appropriate preposition along with which, such as in, at, on, or by.

The candidate gave a speech that criticized the incumbent governor.
The candidate gave a speech in which he criticized the incumbent governor. (criticized the governor in the speech)

He taught us a method that would improve our shooting.
He taught us a method by which we could improve our shooting. (improve our shooting by that method)

This is a case that requires caution.
This is a case in which caution is required. (caution is required in this case)

The expression “[something] is/was when” is in widespread and frequent use, but some grammar authorities disapprove of the use of this expression:

 My favorite part of Main Street is when Carol encounters the town pariah. (“favorite part is when“)

One simple solution is to replace the to be verb with to occur:

My favorite part of Main Street occurs when Carol encounters the town pariah.

Another option is to slightly reword the sentence to use a which clause like those above:

My favorite part of Main Street is the chapter in which Carol encounters the town pariah.

Then, of course, as a writer you always have the option of substantially rewording the sentence to avoid the issue entirely:

My favorite part of Main Street is Carol’s encounter with the town pariah.