affect/effect: Suggested by eisoj5, who is right. People keep misusing these, even professors, even editors. There really is nothing to be done but memorizing them, and then using them again and again until you get them right, damn it. Not just letting them go and figuring that's all right, that they don't matter anyway.
Most of the time, "affect" is the verb and "effect" is the noun. It's not true all the time, but it's true probably 90% of the time.
"She affected him" has to start with a. "That was an effect of his presence" has to start with an e. If it helps, think of "cause and effect," which is a phrase that people rarely misuse due to its being so familiar.
The other 10% of the time, "affect" can be a noun, meaning emotion that's a consequence of something- "She didn't show much affect"- and effect is a verb meaning to create or bring about- "They effected the revolt they had so long dreamed of." But these aren't common, and unless you specifically mean one of them, you want affect as a verb and effect as a noun.
It just has to be memorized.
accommodate: Apparently the most misspelled word in the English language. Two c's, two o's, two m's. The fact that they are not all right next to each other cannot be helped.
lie/lay: NTSFW, these again have to be memorized.
lie (majority of the time): the act of lying down done consciously, as by a person.
lay (majority of the time): the act of lying down done involuntarily, as by a person laying an object down.
They also have other senses, such as a chicken laying eggs, but people rarely confuse those. Those two, they do, but there's no need for confusion as long as you keep that conscious/unconscious distinction in mind.
Parts (present tense, present participle, past tense, past participle):
lie: lie, lying, lay, lain.
lay: lay, laying, laid, laid.
Lie's actually the more confusing one here, since its past tense is the same as the present tense of the other, but people are always turning to "lay" and talking about a cat "laying" on the carpet (shudder). Do try to keep them in mind.
set/sit: NTSFW, and the distinction is similar to the "lie/lay" one. To set something, it has to be an object; humans can't "set" in a chair. You set a table, you set a bomb to go off, and so on. The only exceptions to that are the sun setting and the hen setting on her eggs (the connection of chickens with some of the most confusing verbs in English ought to be investigated someday).
"sit" is usually conscious. It can be done on an object, like a chair, but it's not the chair that sits; it's the human or the animal, who has a conscious choice about it. The only exception I can think of here is "sitting" someone down, which involves someone forcefully grabbed and stuffed into a chair. That involves the action of sitting, so it's still "sit" and not set. Setting someone in a chair would imply that they had no choice about resistance, as if they were unconscious or a baby.
Yeah, while we're on that.
unconscious: Decide what this word means, please. Most of the time, it implies that the character is not paying attention, either through actually having passed out or because she's so focused on something else. But I keep reading that a character was "unconscious" of having heard or seen something, and yet managed to think about it consciously. It's sloppy POV work as well as a sloppy use of the word. Decide what the character is paying attention to and what she is not, and if she really is unconscious of a sound or sight, then don't describe it until it surprises her.
wench: This is not a flattering word. I don't know why some people think it is. It implies lower class and possibly some sluttiness, in that "bar wenches" are often thought to consort with the customers. If you have your low-born sailor use it to your noble lady, he should get slapped.
dialogue tags: I'm a champion of "asked" and "said." They're the simplest, they're nearly invisible to most people (much like "he" and "she"), and they don't imply that your character is doing anything but speaking.
Yes, "asked" and "said" can become boring, but the constant use of "groaned," "yelled," "cried," and "interjected" isn't much better, especially when they're misused.
So, a brief crash course (of course, this is in what I find most offensive):
whisper- should only be used when the character is trying to be quiet or unnoticed. Having them whisper a shout is ridiculous.
groan, moan- have you ever actually tried to hear words in this sound? They might follow one, but they're unlikely to make it through.
hiss- your characters can only hiss something that actually has s's in it.
shout, cry, yell- these are violent words. Don't use them if the character is calm or has no reason to shout, cry, or yell.
interject- this means to put in the middle of something, as done by an interrupting character. If your character isn't interrupting anybody, don't use it.
smile, nod, shrug- should not be used. How do you "nod" words? For that matter, have you tried to grin widely and talk at the exact same time?
Maybe more on this later.