Don't let silly grammar mistakes get in the way of your writing! A few common mistakes are easy to overlook, either because they sound the same (homonyms, like "to" and "too") or because we were staring out the window in English class when the teacher covered it. Take it from a professional copy editor: Your writing will earn a lot more respect if you take time to learn some of the most common mistakes and some hints on how to remember the correct forms. Don't think of this as "grammar policing"; think of it as "grammar whispering"!
Do you remember learning about dependent and independent clauses? An independent clause is a complete thought; by the adjective "independent," you know that it could stand by itself if it wanted to, and may not necessarily relate to the other parts of the sentence. A dependent clause, on the other hand, needs a prop. Use "that" when attaching a dependent clause, and "which" when attaching an independent clause.
Take this sentence: "The cat that climbed the tree was orange and white." In this sentence, the phrase "that climbed the tree" is a dependent clause and tells you that you are considering a particular cat — the one that climbed the tree. You use "that" to point out this particular cat. However, if you write "The cat, which was orange and white, climbed the tree," you use "which" because this cat just happens to be orange and white; the important thing is the fact that it climbed the tree. (Note that dependent clauses are not set off with commas, but indpendent clauses usually are.)
These two words are frequently confused, but they are very easy to tell apart once you know the trick. Use "then" when you are referring to a time sequence: "I'm going to write an article, and THEN I am going to publish it." A no-fail way to keep them straight is to remember that "then" and "time" both contain an "e."
Use "than" when you are comparing two things: "This article is longer THAN the one I wrote yesterday." Remember this by noticing that "than" and "comparing" both contain "a."
Write with one "o" when it is a preposition, which always refers to a position or direction (to the gym, to the table, to the moon) or part of an infinitive verb form (to shop, to write, to publish). Add another "o" and you're implying that there is much more of something, such as "too many adjectives" or "too much lipstick." It also means the same as "also"; "Wait for me; I'm coming too!"
I'm sure you've heard this a zillion times, but since the word "unique" means "one of a kind," something cannot be "very unique." Don't add any adjectives to "unique"; let it stand on its own. If you really must have an adjective (and don't we all sometimes?) say "unusual"; you can add any adjective you want to that word!
Even professional journalists get this wrong sometimes; I once heard a reporter on NPR talk about protesters "laying down" in front of trucks. Some grammarians say that that's okay, because the English language is a living thing that is changing all the time, and it will eventually reflect how people actually talk rather than follow some dusty rules that people made up a long time ago. But if you want to be scrupulously correct, say "lay" when you are describing an action done to an object, such as "Lay the book on the table"; and "lie" when you are talking about a person or animal going horizontal, such as "I'm going to lie down and take a nap." (Hint: if you're going to follow the verb with "down," the correct choice is almost always "lie." You CAN say "lay my body" or, as in the children's prayer, "Now I lay me down to sleep" — but that's just confusing.)
A lot of us get tripped up when we are including ourselves with someone else in a sentence. We all know that it's polite to say, "David and I went to a movie," but what if you and David were invited out with friends? It's tempting to say "The Jetsons went to a movie with David and I" — but that would be wrong. Take out David and you would easily, and correctly, say, "The Jetsons went to a movie with me." Don't change it if you add David back in; it's correct to say, "The Jetsons went to a movie with David and me."