The Dower Power Hour

Who versus Whom
If you could replace “who” with “she,” it’s who. If you could replace “whom” with “her,” it’s whom.
Mrs. Dimwit consulted an astrologer whom she met in Seattle. (She met him in Seattle.)
Joyce is the girl who got the job. (She got the job.)

Which versus That
If you could put it in parenthetical, it’s “which.” If you couldn’t, it’s “that.”
The bicycle, which I couldn’t find…
The bicycle (which I couldn’t find)… (Works.)

The bicycle that I like was lost.
The bicycle (that I like) was lost. (Wrong, doesn’t work.)

Court is singular.
When the only basis for the federal court’s subject matter jurisdiction over a matter is diversity of citizenship, it must apply state substantive rules. (Correct).
When the only basis for the federal court’s subject matter jurisdiction over a matter is diversity of citizenship, they must apply state substantive rules. (Incorrect)

Their versus His or her
In writing research and other academic works, it is important to consider the reader and try to grab and maintain their interest. (Incorrect)
In writing research and other academic works, it is important to consider the reader and try to grab and maintain his or her interest. (Correct)
In writing research and other academic works, it is important to consider the readers and try to grab and maintain their interest. (Correct)

Affect versus Effect
Affect = Verb
Effect = Noun

The drug did not affect me.
The drug had no effect on me.

Note: Affect can be a noun, as used in psychology.
Note: Effect can be a verb, such as “effect change.”