Unconfounding the after

How do you tell if words such as after, before, and until are subordinating conjunctions or prepositional phrases?

IF "AFTER" IS FOLLOWED BY A CLAUSE (i.e. includes subject and verb), IT IS A SUBORDINATING CONJUNCTION. IF "AFTER" IS FOLLOWED BY AN OBJECT, IT IS A PREPOSITIONAL PHRASE.

PREPOSITIONAL PHRASE

• Charlie will wait here until sunset. ("until sunset" acts as an adverb)
• It will be some time before summer. ("before summer" acts as an adjective)
• After that effort, everyone doubts whether she can win. ("after that effort" acts as an adverb)

SUBORDINATING CONJUNCTION

• Charlie will wait here until we finish the test.
• It will be some time before the seasons change.
• After she did so badly, everyone doubted whether she could win.

Source for examples: The Tongue Untied