A couple more words I see people mixing up all the time:

Averse and Adverse

While similar in some ways, they are not interchangeable. First, let’s look at averse.

According to Merriam-Webster online:

Function: adjective
Etymology: Latin aversus, past participle of avertere
Date: 1597

: having an active feeling of repugnance or distaste —usually used with to; averse to strenuous exercise

Ok, so it’s an adjective most of the time, such as in the usage above. This is when you don’t want to do something, like you’re averse to going to work on Monday morning (welcome to my world).

Now, adverse, again from Merriam-Webster online:

Function: adjective
Etymology: Middle English, from Anglo-French advers, from Latin adversus, past participle of advertere
Date: 14th century

1 : acting against or in a contrary direction : hostile
2 a : opposed to one’s interests (an adverse verdict) (heard testimony adverse to their position); especially : unfavorable (adverse criticism) b : causing harm : harmful (adverse drug effects)
3 archaic : opposite in position

Also an adjective, but more in the sense of something outside you being bad for you, working against what you want.

The first is more what you feel about something, the second shows how something affects you (adversely).
Like your girlfriend will probably have an adverse reaction when you tell her you want to break up with her and start seeing her best friend. Good luck with that. Most people are averse to being dumped.

Of/Have

This is one I’ve started seeing used incorrectly fairly often in print. Confusion around the use of these two in phrases such as ‘should have,’ ‘would have,’ ‘could have,’ no doubt arises from the contracted forms:

should’ve
would’ve
could’ve

Now I see in print “I should of” or “We could of” when no doubt the writer meant “I should’ve” or “We could’ve.” They sound the same when you say them, but they are completely different. “Of” is a preposition (I know, we’re getting technical), whereas ‘have’ is one of the three helping verbs: have, be, and do.

Let’s just try substituting one for the other in a less ambiguous context. For instance: You wouldn’t say “We of been here for an hour.” You would say “We have been here.” Similarly, you couldn’t say “She thought have him,” but you would say “she thought of him.” Hopefully this shows that they’re not interchangeable.

Lose/Loose

Under most circumstances, one is a verb, the other is an adjective. “Lose” is the verb. You can lose your keys, lose your mind, lose your lunch. If you lose something, it’s lost. Until it’s found.

Loose is, most often, an adjective. Loose rhymes with goose. You can have a screw loose, in which case you might lose it. If you lose weight your clothes will feel loose (how nice).

There are other uses for each word that you can look up in any dictionary of your choosing, but these are the most common situations where these two get mixed up.

I wanted to point out a couple of things that I constantly hear, from people who should know better. Without getting too technical and invoking a lot of grammar jargon like predicate and participial I’ll just briefly mention a few things I hear most often.

He/Him, She/Her

What I hear is “Him and I talked…” or “Me and her met…” I hear it at work, on television (you’d think the writers of the shows would know better, wouldn’t you?) This does you no credit, because you’re bound to end up working for someone who knows better and will cringe everytime you say something like that.

The word “and” is a signal that both halves of that conversation are the subject. Here’s what you can try: Remove the “and” along with either the first or second party, and what do you have? “Him talked…” or “Me met,” or “Her met.” I know no one would say those things, but for some reason when they’re together, people can’t see it. Simply put, whichever pronoun you use in front of and after the “and” has to be able to stand on its own, as if the second one wasn’t there.

Correct: “He and I talked” or “She and I met”

The Death of the Infinitive “To Be”

Another trend I’ve noticed over the last several years is that of dropping the verb “to be,” as in “I need briefed,” or “He needed taught.” It makes it sound as if “briefed” and “taught” somehow became nouns, things you could pick up at the grocery store. This is not just slang people use in informal conversation with friends, I hear this from degreed professionals in the office. How did this happen? Where did this start? The first time I heard it was from a guest speaker at a convention who uttered the first example phrase, “I need briefed.” I’ve been scratching my head over it ever since, and that was probably 20 years ago, and the usage seems to be picking up steam over time. It’s such a tiny little verb, maybe that’s why it gets overlooked? It may be trendy, but it sounds, and is, wrong.

Than/Then

Now this one I see in print All.The.Time. Most often I see then used when it should be than. ‘Than’ is comparative, as in “Steve is taller than John,” or “Marcy’s cookies were better than Sue’s.”

‘Then’ is used in time situations. “If he arrives on time, then we’ll go to the movie,” which specifies “That is when” the event will occur. Maybe the ‘e’ in ‘when’ and ‘then’ can work as a little mnemonic?

Went/Gone

Wrong: Had went.
Right: Had gone.

Go-Went-Gone. I go. I went. I had gone.

There are more I could get into but if you have questions, here’s a great site which can help with a quick question, and minimal, but effective, explanations: Common Errors in English Usage.