In this post, I'll highlight the ones I saw today in some comments posted to news items I read over my morning coffee.
You, Your, You're
You is a pronoun that can be singular or plural (one of those English language things that make it difficult to learn).
The possessive form of a pronoun is always formed without an apostrophe, unlike most nouns. The possessive of you is your, not you're.
You're is a contraction for you are. For example, “You're the one that I want.”
Right: You're going to need your thinking cap if you are going to understand this.
Wrong: Your stupid if you don't know you're pronouns, possessives, and contractions.
It, Its, and It's
This trio gives many people trouble. I remember one day debating with my boss about the possessive of “it”. He was certain it had to be it's based on the rule that to make a possessive you add an apostrophe and s. This is true of most nouns but never with pronouns.
It is a third-person singular pronoun generally used to refer to inanimate things, or when the gender of the living thing isn't known. For example, “I ate the apple because it tasted good.” Or, “There is a skunk in the woodpile and it stinks.” (The other third-person singular pronouns (subjective case) are gender specific: he and she.
Its is the possessive form of it. There is no apostrophe. “Everything is in its place.” “The bear returned to its cave.”
It's is a contraction meaning it is. Like all contractions, the apostrophe indicates that two words are joined and some parts of the two are not shown. “It's a good thing that I'm so smart!”
These two are enough for now. I'll write about other pronouns, possessives, and contractions in another post. But feel free to do some research on your own. Wikipedia has a nice pronoun chart that you may find useful.