Connecting Sentences with Conjunctions

When you join two complete sentences (independent clauses) using a coordinating conjunction—like and, but, or, so, or yet—you must place a comma before the conjunction. This tells the reader that a new complete thought is starting.

✅ I washed the dishes, but I forgot to take out the trash. ❌ I washed the dishes but I forgot to take out the trash.

However, if the second part of the sentence is just a second verb and doesn't have its own subject, it is a compound predicate, not a compound sentence. In this case, do not use a comma.

✅ I chased the furry thief for ten minutes but couldn't catch it. ❌ I chased the furry thief for ten minutes, but couldn't catch it.

Setting Off Introductory Clauses

When you start a sentence with a dependent clause or a time word, use a comma to signal where the introductory part ends and the main sentence begins.

✅ Before we even reached the hotel, my suitcase fell into a canal. ❌ Before we even reached the hotel my suitcase fell into a canal.

If you flip the sentence so the dependent clause comes at the end, the comma is usually dropped because the main idea is already established.

✅ My suitcase fell into a canal before we even reached the hotel.

Extra Information and Direct Address

Commas act like parentheses to set off non-essential information. An appositive is a noun or phrase that renames the noun right beside it. If you can remove that phrase without changing the core meaning of the sentence, it needs to be framed with commas.

✅ Barnaby, a fluffy orange cat, thinks he owns the local bakery. ❌ Barnaby a fluffy orange cat thinks he owns the local bakery.

Similarly, when you are talking directly to someone, their name (or title) must be separated by commas, no matter where it appears in the sentence.

✅ I am begging you, Dave, to finally clean up the living room. ✅ Sarah, I fed the cat already.

The Comma Splice Trap

The most common mistake learners make with commas is the "comma splice." This happens when you try to glue two complete sentences together using only a comma, without a conjunction. Commas are not strong enough to hold two independent clauses together on their own.

❌ A squirrel stole my homework, it ran up a giant oak tree.

To fix a comma splice, you have three options: add a coordinating conjunction, change the comma to a semicolon, or split the clauses into two separate sentences.

✅ A squirrel stole my homework, and it ran up a giant oak tree. ✅ A squirrel stole my homework. It ran up a giant oak tree.