Don’t worry. I hated grade school grammar as much as the next guy. Still, as a magazine editor and, later, as a freelance book editor and (now) literary agent, I have come across far too many grammatical and usage mistakes in writing submitted to me. Not all of us can be Strunk or White (though every writer should own their valuable book, The Elements of Style). But we can profit from a little attention to the most common trouble spots:

1. It’s and its. It’s (with an apostrophe) always means “it is,” and its is the possessive form of it (“its jaws gaped”). This is easy to remember; the apostrophe indicates a contraction, so remember what it contracts.
2. Affect and effect. Affect is always a verb meaning to “cause a response” or to “pretend or assume a manner.” Effect can be a noun (a result or accomplishment) or a verb meaning “to bring about” (“the new manager will effect many changes”).
3. Was and were. Of course it’s I was/you were/we were. But a problem usually arises between the subjunctive and indicative moods (yeah, I don’t know what that means, either). I’ll let Harry Shaw explain, in his Dictionary of Problem Words and Expressions: “Use were (the subjunctive), not was (the indicative), in such sentences as: ‘Suppose he were to arrive now’ (supposition). ‘He drank ale as if it were going to be prohibited forever’ (an improbable condition). ‘Roberta wishes that she were going to be invited’ (desire).” I wish it were a little easier, but there it is.
4. Possessive forms. With the exception of Jesus’ and (sometimes) Moses’, when a singular noun ends in “s,” add ’s (Charles’s friend, Burns’s poems).
5. Agreement between nouns and pronouns. One of the most common usage errors I see is the use of a plural pronoun which has a singular antecedent (e.g., “If the customer insists on a refund, they should be given one” should be: “If the customer insists on a refund, he or she should be given one” or “If customers insist on refunds, they should be given them”). I know, some people say that “the customer/they” is okay nowadays, but they are wrong. All of them.
6. Subject/verb agreement. If the subject is singular, the verb must be singular—no matter how many words are between them. Strunk and White use the example: “The bittersweet flavor of youth—its trials, its joys, its adventures, its challenges—is not soon forgotten.” Just take out the interrupting phrase and the correct word becomes clear.
7. A participial phrase at the beginning of a sentence. For example, “Walking slowly down the road, a tree loomed in front of him” should be changed to something like “Walking slowly down the road, he saw a tree looming in front of him.” The participial phrase should be related clearly to its subject.
8. Parallelism. The sentence, “My objections are, first, the injustice of the measure; second, that it is unconstitutional” should become “My objections are, first, that the measure is unjust; second, that it is unconstitutional” (Strunk,p. 28). Or “My objections are, first, the injustice of the measure; second, its unconstitutionality.” Yeah, Strunk’s version is better.
9. The word only. The placement of the word only is often a danger spot. For example, in the sentence, “I only wanted an ice cream cone,” the word modifies wanted. It indicates that I didn’t need an ice cream cone; I only wanted it. But the sentence, “I wanted only an ice cream cone” (with only modifying ice cream cone) indicates that I didn’t want more than an ice cream cone—certainly not an ice cream cone and a float as well. In the sentence, “Only I wanted an ice cream cone,” only modifies the word I. Thus, the sentence indicates that only I—no one else—wanted an ice cream cone. If only people would keep this straight.
10. Accept and except. Accept means “receive” or “agree with” (“I accept your proposal”). As a verb, except means to omit or exclude (“We’ve been excepted from the list”); as a preposition, it means “other than” (“Everyone except me knew the answer”). Except should never be a conjunction (“I won’t go except you go, too”), except in dialogue.
11. Alright and All right. Alright is not a word. It is not all right to use alright. All right?
12. Feel and think. Current usage often uses “feel” rather than “think” (as in, “I feel our president is doing a good job”). However, they are not interchangeable. Feel indicates emotion, think suggests reason (“I feel grateful to have you as my mentor, because I think you have a lot to teach me”).
13. Imply and infer. Shaw writes, “To imply is to suggest a meaning only hinted at, not explicitly stated. To infer is to draw a conclusion from statements, evidence, or circumstances.” Speakers imply; listeners (or observers) infer.
14. Lie and lay. To quote “Dear Abby,” “To ‘lay’ means to set or put; to ‘lie’ means to recline. Remember, chickens lay eggs. People lie down.” So it’s “lie/lay/lain/lying,” and “lay/ laid/laid/laying.” I ain’t lying.
15. Fewer and less. Can you count the items you’re writing about? If so, use “fewer.” If the quantity is not countable (referring to volume rather than number), use “less.” For example, you can have fewer ingredients, coins, or puppies, but less water, money, or love.
16. “One of the only.” I must conclude with a pet peeve of mine. Strictly speaking, “only” means just one in number. So, when you write, “It was one of the only times I felt that way,” you should write, “one of the few times.” Unless you mean there were no other times, in which case you should write, “It was the only time I felt that way.” This is the only time I’m going to say this.