So first, you’ll need to know the difference between clauses and phrases. If you don’t know, don’t worry–it’s actually quite easy.
A clause has a subject and a verb (predicate), just like a sentence. In fact, a clause can be a sentence. We covered that in Commas and Independent Clauses.
A phrase lacks the subject-predicate form. Some of them don’t even have verbs. By themselves, they’re not complete sentences.
Use a comma after introductory clauses.
Introductory clauses are dependent. They introduce the main, independent clause; they set the stage for the real sentence. Look for words like: after, although, as, because, before, if, since, though, until, when, and while.
- After he got the Ring to Rivendell, Frodo still had to take the sorry thing to Mordor. In this case, after he got the Ring to Rivendell is a clause that introduces or sets the stage for the real sentence, Frodo still had to take the sorry thing to Mordor.
- Although he never wanted to leave the Shire, Sam still hauled himself to the Cracks of Doom.
- Because the Ring was inherently evil, they couldn’t even it ditch it with Galadriel or Gandalf.
- If they had, they would’ve only ended up with a new Dark Lord.
If you lop off the first word in the dependent clauses above, what is left is a subject-predicate sentence.
However, this rule concerning clause only works if they introduce the sentence. If you rearrange the sentence so the dependent clause comes at the end, you no longer need a comma.
- Frodo still had to take the sorry thing to Mordor after he got the Ring to Rivendell.
- They couldn’t even ditch it with Galadriel or Gandalf because the Ring was inherently evil.
- They would’ve only ended up with a new Dark Lord if they had.
The single exception to this is in cases of what Purdue calls ‘extreme contrast.’ https://owl.english.purdue.edu/owl/owlprint/607/
The example they give is: She was still upset, although she had won the Oscar.
I don’t know about extreme, but although is a word that denotes contrast. I’d still put it in in my example:
- Sam still hauled himself to the Cracks of Doom, although he never wanted to leave the Shire.
If you’re not looking to contrast something, you’ll probably want to pick another word. I’m sure someone can find exceptions, but generally, I put a comma before although.
Got that? Good. Make sure you use a comma after introductory clauses. Introductory phrases will get a whole post of their own because there are so many of them.