I know this post will invite tons of scrutinizing to everything I write, but I'm a teacher and can't keep quiet when very basic rules of the English language are completely ignored.
1) Apostrophes are used to show possession. For example, "The woman's red pen ran out and she had to use purple which made her feel silly." If you want to make a noun plural, as in show that there is more than one, you just add an 's' in most cases. For example, "The teachers discussed the topic of timing in lessons for the whole 40 minute meeting when there were 5 topics on the agenda." You can also use an apostrophe in a contraction, like the example above (on the sweatshirt), but I rarely see that one misused.
2) They're means they are, as in, "They're going to drive me totally insane with all this extra testing and observations." Their is possessive, as in, "It is their job, I get it, but I used to love mine and now I'm not so sure." There is a place, as in, "Can we go over there so I can have a few drinks instead of grading tests I didn't even want to give?"
3) It's is the contraction for it is, as in, "It's going to rain today, which means the students will be bouncing off the walls in seventeen different directions." Its is possessive, as in, "Its heart is beating, so the kid must be alive, but she has slept through the entire class and won't leave now that school is over."
What are your biggest pet peeves? My husband's is that I always correct his spelling and pronunciation...