People who care about grammar care about it a lot. And for everyone who cares about grammar (and even some people who don't), there are certain misspellings, wrong usages, and grammatical errors that just set the teeth on edge.
Think you don't need to pay attention? One of those people may be your boss, your investor, or your biggest customer. Most won't correct you when you make a mistake that drives them crazy, they'll just look away and stay silent.
Even smart people and seasoned writers get some of these wrong. Take a look at this list and see how many of these common words and phrases you're misusing and need to correct in your own writing:
1. It's vs. its
This is one that drives me up the wall and it's very, very, very common. It's = it is. Its = belonging to it. Please, please stop putting in the apostrophe when using "its" in the possessive sense.
2. That vs. who
"That" refers only to inanimate objects. So this is wrong: "Mary was the one that wanted us to meet today." No, she was the one who wanted you to meet.
3. That vs. which
The difference between these is "that" refers to a defining condition and "which" refers to a non-defining condition. So if someone is trying to gather items for the laundry, you would say, "The shirt that is dirty is on the floor." If someone wants to borrow your shirt, you would say, "The shirt, which is dirty, is on the floor."
4. Between vs. among
"Between" refers to a choice between specific objects (or people), usually two but sometimes more. "Among" refers to a group. "I couldn't find the right shirt among the many shirts in the laundry." Or: "I couldn't choose between the two shirts he offered me."
5. Alright vs. all right
The word "alright" is much debated in grammatical circles. There are many who believe that it is always wrong and "all right" is the only proper usage. Others say "alright" is all right. Why take chances? Use "all right" unless it's an exclamation, say, in response to someone making a basket or landing a big sale.
6. Affect vs. effect
Using "effect" as a verb meaning "to change" is not only wrong, it will set some people's teeth on edge. To change something is to affect it or have an effect on it. "Effect" can be used as a verb, but it means "make something happen." People talk about "effecting real change," which adds to the confusion.
7. Lie vs. lay
Anyone who's ever told a dog to "Go lay down!" was misusing the verb. "Go lie down!" would be correct. "Lay" is transitive, which is a fancy way of saying that a verb needs an object. So you lay the table or lay out a plan, or even lay me down to sleep, but you don't lay down. Unfortunately, "lay" is also the past tense of "lie," which makes this harder to get right than it should be.
8. Free rein vs. free reign
If someone has complete authority to do what they want in a given situation, that's "free rein." As in, when you drop the reins and let the horse take you where it wants to go. I can understand why people think you're giving someone "free reign"--making them monarch--but that's not right.
A representative of a city once told me that his town was "literally exploding" with new business. I--barely--swallowed the impulse to ask if anyone was killed or injured. Please, please don't use "literally" unless you mean it, well, literally.
10. Should of, could of
This error probably has its roots in the contractions "should've" and "could've," which are replacements for "should have" and "could have." When spoken, "should of" and "could of" sound right because they sound like those contractions. They're not. When you're writing, use "should have" and "could have."
11. Peak vs. pique
I'm always getting pitches where someone says they want to "peak" my interest about something. I get why that seems to make sense--they want to bring my interest to the highest possible point. But no, the phrase is "pique" one's interest, meaning to stimulate or provoke.
12. Pour vs. pore
Similarly, I often read that someone "poured over" a text. Only if they spilled their drink. The correct term is to "pore over," meaning to be deeply absorbed in the study of something.
13. I vs. me
There are many times when "I" is correct. Those times are when it is the subject of a verb. For instance: "Joe and I went to the store." Mistakes arise when people who've been corrected from saying "Joe and me went to the store," start using I everywhere. As in, "They were happy to see Joe and I." When used as the object rather than the subject of a verb, use "me," not "I".
Not all grammar geeks agree about everything, and this is a word that's been debated quite a bit. Many people believe the term should be used only in a reflexive sense, where "I" comes up more than once, such as "I bought myself a car." Your safest bet is to stick to those usages.
15. Less vs. fewer
"Less" refers to an amount of something, such as, "There was less water in the pool than there was before." If you're talking about a number of something, use "fewer." "There were fewer people in the pool because the water was low." "Less people" is wrong.
16. Lose vs. loose
These two words get switched around all the time. "Loose" is the opposite of tight. As a verb, it means to release, as in "he loosed the hounds." If you're referring to something you once had but don't anymore, use "lose."
Never use this--there is no such word. If you ever feel the impulse to use it, switch in "regardless" instead.
18. They as a singular pronoun
"They" is supposed to be a plural pronoun, as in "if people arrive late they may be hungry." Traditionally, it's been considered wrong to use it in a singular form, as in "if someone arrives late they may be hungry." Unfortunately, our language doesn't provide a gender-neutral pronoun, and a lot of women (including me) dislike the archaic concept of using "he" to mean "he or she." On the other hand, "if someone arrives late, he or she may be hungry" is awkward. Sadly, your best bet is to dodge the problem by recasting your sentence in a more plural form, or finding a way to avoid the pronoun, such as "someone who arrives late may be hungry."
"Irony" is another much-debated concept among grammarians. There have been lengthy discussions of the Alanis Morisette song "Isn't it Ironic," which lists many situations that aren't actually ironic, such as rain at a wedding. Purists abide by the original definition of "irony" as something similar to sarcasm: What's said is the opposite of what's meant. But I like George Carlin's definition that takes in odd or fitting coincidences this way: If a diabetic is run over by a truck on the way to buy insulin, that's an accident. If the truck was delivering insulin, that's ironic.