subordinate conjunction / preposition
A word that introduces a clause, versus a word that takes a noun phrase.
A subordinating conjunction introduces a clause: because the gate jammed; although the hall was cold. A preposition takes a noun phrase: after lunch; before the vote. Some words do both, such as after, before, since, and until. After a preposition, use a noun or a gerund: after filing. After a subordinating conjunction, use a clause: after she filed.
Examples
UseAfter lunch, we left. (preposition + noun)
UseAfter she signed, we left. (conjunction + clause)