Continual and Continuous

They’re similar, but there’s a difference. “Continual” means something that's always occurring, with obvious lapses in time. “Continuous” means something continues without any stops or gaps in between. e.g., The continual music next door made it the worst night of studying ever. e.g., Her continuous talking prevented him from concentrating.

Envy and Jealousy

The word “envy” implies a longing for someone else’s good fortunes. “Jealousy” is far more nefarious. It’s a fear of rivalry, often present in sexual situations. “Envy” is when you covet your friend’s good looks. “Jealousy” is what happens when your significant other swoons over your good-looking friend.

Nor

"Nor” expresses a negative condition. It literally means "and not." You’re obligated to use the “nor” form if your sentence expresses a negative and follows it with another negative condition. “Neither the men nor the women were drunk” is a correct sentence because “nor” expresses that the women held the same negative condition as the men. The old rule is that “nor” typically follows “neither,” and “or” follows “either.” However, if neither “either” nor “neither” is used in a sentence, you should use “nor” to express a second negative, as long as the second negative is a verb. If the second negative is a noun, adjective, or adverb, you would use “or,” because the initial negative transfers to all conditions. e.g., He won’t eat broccoli or asparagus. The negative condition expressing the first noun (broccoli) is also used for the second (asparagus).

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.