The words after and before are often used as subordinating conjunctions. You can begin a complex sentence with either of these two words:
Before we eat dinner, I have to fire up the grill.
After the rain stops, we can go to the store.
Each of these sentences begins with subordinating conjunctions Before and After. Each of the subordinate clauses contains a verb and shows action.
The words before and after can also be used as an introductory preposition.
Before dinner, we can play volleyball.
After work, I will wash the car.
Notice that these are simple sentences, not complex sentences.
There is no action associated with the words, and thus they are not part of a subordinate clause.
Before dinner and After work are merely phrases.. However, we do place a comma after each of the introductory phrases, just as we do with a subordinate clause.
The words before and after can be used as subordinating conjunctions or prepositions. When you begin a sentence with the words before or after, you will need to use a comma after the subordinate clause or the prepositional phrase.
▶It’s your turn. Place a comma in the correct location in this sentence. After school we will play basketball. Did you put the comma after the prepositional phrase? Good for you!