This post first appeared on Business Insider on 25th March 2017. 

Grammar rules can seem like a nuisance. Honestly, do you really need to check every single document for appropriate hyphenation?

According to CUNY Journalism Press editor and writing coach Timothy Harper, the answer is a resounding “YES.”

“The whole point of grammar and punctuation is clarity,” he told Business Insider. If you write that a woman has “dirty-blonde hair,” for example, people know that you’re referring to the colour. “It doesn’t mean that she needs a shampoo,” Harper said, which it would if you’d written “dirty blonde hair.”

We asked Harper about the most common grammar mistakes he sees and added some that drive us crazy on a daily basis. Read on for a list of tricky – but super important – rules that get broken way too often.

1. Confusing ‘fewer’ and ‘less’

Harper said he winds up correcting this mistake pretty often.

He explained that “fewer” is appropriate when you’re discussing countable objects. On the other hand, “less” is appropriate when you can’t count the thing you’re describing.

Here’s an example of each word in a sentence:

“Fewer than 20 employees attended the meeting.”

“I spent less than one hour finishing this report.”

2. Confusing ‘amount’ and ‘number’

Again, it’s a question of whether you can count the thing you’re describing.

Harper gave examples of how you might use each word:

“There is a really large number of books in that library”

“There’s a huge amount of water going over the dam right now.”

3. Confusing ‘it’s’ and ‘its’

Normally, an apostrophe symbolizes possession. As in, “I took the dog’s bone.” But because apostrophes also usually replace omitted letters – like “don’t” – the “it’s” vs. “its” decision gets complicated.

Use “its” as the possessive pronoun: “I took its bone.” For the shortened version of “it is” use the version with the apostrophe. As in, “it’s raining.”

4. Confusing ‘who’ and ‘whom’

When considering whether to use “who” or “whom,” you have to rearrange the sentence in your head.

So the question, “Whom did you give the letter to?” changes to “You gave the letter to whom?” “Whom” suits the sentence instead of “who” because the word functions as the object of the sentence, not the subject.

It’s not always easy to tell subjects from objects but to use an over-simplified yet good, general rule: Subjects start sentences (or clauses), and objects end them. In short, who does it to whom.

For reference, “Who is a hypocrite?” would be a perfectly grammatically correct question to ask.

5. Confusing ‘him’ and ‘he’

Harper said he often hears people say something like, “Him and me went somewhere.” That’s incorrect. Instead, you should say, “He and I went somewhere.”

Things get slightly more confusing from here. It’s incorrect to say, “He gave it to she and I.” Instead, you should say, “He gave it to her and me.”

If you’re having trouble with this rule, Harper suggested taking away the “and.” For example, you can probably tell that the sentence “He gave it to I” sounds weird, so you can figure out that “He gave it to she and I” is also incorrect.

Read the remaining grammar mistakes by Business Insider here