Do I need a comma after a subordinate clause?
A subordinate clause adds context to a sentence. It is not a proper sentence on its own.
A subordinate clause starts with a subordinate conjunction. Common subordinate conjunctions are: after,
in order that,
In the following sentences, we have put the subordinate clauses in bold.
Julia chose to visit the museum since she's an art major.
Since Julia is an art major, she chose to visit the museum.
If you go to the museum, you will learn something about history.
You will learn something about history if you go to the museum.
From these examples, you can see that subordinate clauses can either come at the beginning or end of a sentence. When subordinate clauses come at the end of the sentence, you don't use a comma. However, when a subordinate clause comes at the beginning of the sentence, you set it off with a comma.
Be careful, though: some subordinate conjunctions can also act in other roles. For instance, the word 'that' can also be a determiner, e.g. That car is red.