Here are a few of those tricky words that often cause us grief as we write, courtesy of Jon Gingerich.
May and Might
“May” implies a possibility. “Might” implies far more uncertainty. “You may get drunk if you have two shots in ten minutes” implies a real possibility of drunkenness. “You might get a ticket if you operate a tug boat while drunk” implies a possibility that is far more remote. Someone who says “I may have more wine” could mean he/she doesn't want more wine right now, or that he/she “might” not want any at all. Given the speaker’s indecision on the matter, “might” would be correct.
Whether and If
Many writers seem to assume that “whether” is interchangeable with “if." It isn’t. “Whether” expresses a condition where there are two or more alternatives. “If” expresses a condition where there are no alternatives. e.g., I don’t know whether I’ll get drunk tonight. e.g., I can get drunk tonight if I have money for booze.
Fewer and Less
“Less” is reserved for hypothetical quantities. “Few” and “fewer” are for things you can quantify. e.g., The firm has fewer than ten employees. e.g., The firm is less successful now that we have only ten employees.
Farther and Further
The word “farther” implies a measurable distance. “Further” should be reserved for abstract lengths you can't always measure. e.g., I threw the ball ten feet farther than Bill. e.g., The financial crisis caused further implications.
Clancy's comment: Mm ... Some words still cause me pain.