9 Punctuation Mistakes You Should Never Make

National Punctuation Day is celebrated every year in the US on September 24.

Founded by Jeff Rubin in 2004, the day was created to celebrate the correct usage of all punctuation, from the “lowly comma” to the “ever-mysterious ellipsis.”

In honor of National Punctuation Day this year, INSIDER rounded up nine basic punctuation mistakes you should never make in formal writing.

Watch out for these errors in your cover letters, academic essays, professional emails, and more.

1. Don’t confuse “it’s” and “its.”

The mistake: Apple will announce it’s new iPhone soon. Its a big year for the company.

Why it’s wrong:“It’s” is a contraction for “it is” or “it has.” “Its,” without an apostrophe, is a possessive pronoun.

Here’s a tip: If you’re unsure about which word to use, just substitute the word with “it is” or “it has.” If the sentence makes sense with the substitution, use “it’s.” If it doesn’t, use “its.”

2. Don’t use an apostrophe to pluralize nouns.

The mistake: Those home’s were built in the 1960’s.

Why it’s wrong: Apostrophes are used to indicate possession and create contractions. They’re also used to pluralize single uppercase or lowercase letters, depending on which style guide you follow.

Here’s a tip: Don’t use apostrophes when you pluralize nouns, numbers, symbols, or multiple letters, such as VIP.

3. Don’t write run-on sentences.

The mistake: I left work early but I missed my train and I waited in the station for 40 minutes until the next one finally came so I didn’t get home until 8 p.m.

Why it’s wrong: Without commas, sentences become a confusing, grammatically incorrect block of text.

Here’s a tip: When linking two independent clauses, use a comma before any coordinating conjunction that connects two independent clauses. An independent clause is a sentence that can stand alone.

Use the acronym FANBOYS (for, and, nor, but, or, yet, so) to remember every coordinating conjunction.

Also, don’t just say your sentence out loud and add commas whenever you pause — you may be inserting comma splices. Instead, memorize these 13 rules for using commas correctly.

4. Don’t overuse commas.

The mistake: I went home , and ate dinner.

Why it’s wrong: Using too many commas can also be grammatically incorrect. For example, a comma splice occurs when you connect two independent clauses with only a comma instead of a comma followed by a coordinating conjunction.

Here’s a tip: Use a comma to link two independent clauses or to connect a dependent clause with an independent clause. A dependent clause is a group of words with a subject and a verb that cannot stand on its own.

In the sentence above, “and ate dinner” doesn’t have a subject so you don’t need to insert a comma before “and.”

5. Don’t use dashes and hyphens interchangeably.

The mistake: I’ll buy the iPhone 8 – if I save up enough money.

Why it’s wrong: A dash ( — ), or em dash or long dash, is most commonly used to indicate a meaningful pause in text or abrupt change in thought.

A hyphen ( – ) is a short line that connects words. Use it to avoid ambiguity or to create compound modifiers before a noun. Compound modifiers consist of two or more words that express a single concept, such as “know-it-all” or “full-time.”

Here’s a tip: Only use a hyphen when you’re connecting two or more words. Also, memorize these rules for using dashes correctly.

6. Don’t use semicolons and commas interchangeably.

The mistake: The traffic was crazy today ; but I arrived at work on time.

Why it’s wrong: Semicolons are most commonly used to separate two independent clauses that are closely related in meaning or subject matter. Compared to a comma or a period, a semicolon emphasizes the relationship between two clauses. For example: I can’t go out tonight; I’m running a marathon tomorrow.

Never use a semicolon to connect two independent clauses that aren’t related.

Here’s a tip: In general, only use a semicolon in places where you could also use a period. You should also never use a semicolon in place of a comma before a coordinating conjunction — that’s what commas are for.

The only exception to these rules is when you’re writing long sentences or complex lists that already have several commas.

For example:

If I want to buy the iPhone 8, I’ll have to start saving today; but if the iPhone 7 gets cheaper, I’ll consider that option, too.

Today’s commute was unbearable: first, I missed my train; then, I waited in the station for 40 minutes; and finally, the train broke down halfway through my trip.

7. Don’t use semicolons and colons interchangeably.

The mistake: I ate three things for lunch ; a sandwich, an apple, and a bag of chips.

Why it’s wrong: Don’t use a semicolon to do a colon’s job. Anything you write after a colon should be a summary, interpretation, or elaboration of what came before the colon. Colons can also be used to add emphasis and introduce dialogue or quotes.

Here’s a tip: Generally, the first word after a colon is not capitalized. Only capitalize the first word if it is a proper noun, the start of a complete sentence, or the start of at least two complete sentences or a direct question, depending on which style guide you follow.

8. Don’t place periods and commas outside quotation marks (in the US).

The mistake: “The train was delayed for 40 minutes”, the anonymous source told INSIDER.

Why it’s wrong: According to style guides in the US, periods and commas must be placed inside quotation marks. If you follow British style guides, punctuation is placed outside quotation marks.

Here’s a tip: Always place periods and commas inside quotation marks. Place all other punctuation — colons, semicolons, question marks, etc. — outside quotation marks, unless they are part of what is being quoted, such as a quoted question.

9. Don’t overuse exclamation marks.

The mistake: The iPhone 8 will reportedly cost $999 ! I can’t believe it ! I also can’t afford it !

Why it’s wrong: You should avoid overusing exclamation marks in formal writing, especially in academic essays. Using too many exclamation marks can overwhelm the reader and undermine the emotion behind each individual mark.

Here’s a tip: If you want to be taken seriously in an academic or professional setting, scale (all the way) back on your use of exclamation marks. It’s always better to let your writing, and not your punctuation, evoke emotion.

This article originally appeared on BusinessInsider.com