In The Grapes of Wrath, when is humour used?I am having trouble coming across quotes or occurances that show humour.
This is an excellent question, and your struggle to find humor in this story mirrors the Joads' inability to do the same. Things must be pretty bad if there is no humor to be found.
Now, that being said, there is some minimal humor in the novel, i think, primarily found through certain characters. Granma and Grampa are quite humorous, both in what they say and what they do. Granma has her favorite sayings, but Grampa is the real cut-up--though probably not on purpose. He's always unbuttoned or dribbling or cursing...not unlike some older people we probably all know.
How about the fanatical religious woman who acts so ridiculously and finally ends up being carried away by some of the boys. Uncle John even has a few humorous incidents, though we're laughing at him while he finds the situation anything but amusing.
That should get you started thinking about humor in perhaps another context. It's rather dark humor, but it's a rather somber novel and anything more outrageous would be out of place and almost obscene.