A subordinating conjunction introduced a clause. (Please note, a clause has tense)Originally Posted by spot
Conjunction: The farmer gasped after he had killed the goose.
'after', a subordinating conjunction, introduces the clause 'he had killed the goose'; note, 'had killed' carries tense.
If 'after' doesn't introduce a clause, then it's not a conjunction:
Preposition: After killing the goose, the farmer gasped.
'after', a preposition in form, heads the adverbial phrase 'After killing the goose'; note, 'killing' is not a verb.
Both the phrase and the clause are adverbial: they answers the question, When?
Q: When did the farmer gasp?
A: After killing the goose. (adverbial phrase)
A: After he had killed the goose. (adverbial clause)
In short, subordinating conjunctions join clauses. So when in doubt, look for a tense-carrying verb.