* I always ask students, which is right: “I can’t wait ’til tomorrow” or “I can’t wait till tomorrow”? They always answer the first one and they’re always shocked to learn that the preferred form is the second one. As a shorter alternative to “until,” the preferred form is till, not ’til, because till is a synonym for until that actually predates it. Therefore, there’s no need to shorten until. (Indeed, in-the-know folks consider this a point that separates themselves from less-savvy writers, such as the people behind those “No payments ’til January” ad copywriters.)
* The reason “I feel bad” is usually the correct choice over “I feel badly” is the same reason we say “I feel happy” instead of “I feel happily.” The concept is that of linking verbs, also called copular verbs. These special verbs take adjectives, not adverbs, as their complements.
* I always ask students: Which is right: "In school I got As and Bs” or “A’s and B’s” or “A’s and Bs”? They always answer the first or second one. None of them ever guesses the third or the possibility that all are correct. But they are all acceptable.
In fact, the third example, which uses an apostrophe in A’s but none in Bs is taken straight off the front page of the Los Angeles Times. It's their style. The reason: Apostrophes, which most often form possessives or denote omissions (as in contractions), can also be used “to avoid confusion.” With the letter grade of B, adding an S does not form a new word. But add S to the letter A, and you have a new word: As. That’s why all three of these choices are valid options.