Actually, -ing forms have two functions:Originally Posted by X Mode
1) as a present participle (i.e., an adjective or a part of a verb)
2) as a gerund (i.e., a noun)
By the way, I understand the usage of 'reduce' now. It means, shorten (i.e., shorten the clause), right? OK.
Change a clause into a phrase when the TIME referred to is obvious from the rest of the sentence. For example,
EX: After he ate, he stepped outside. :D
Since the person who ate and the person who stepped outside are one and the same (i.e., he / he), reducing the independent clause to a phrase is matter of deleting the redundant item, right? Let's look at how "After he ate,...", a clause, becomes "After eating,...", a phrase:
Step 1. INPUT: After he ate, he stepped outside.
Step 2. DELETE SUBJECT: After____ate, he stepped outside.
Note, since all verbs require a subject--overt or covert--, we're going to have to change the form of the verb 'ate' so that it doesn't require a subject. That is we are going to have to take away its tense: ate => eating; having eaten
Step 3. WORD CHANGE: After eating, he stepped outside.
Step 4. OUTPUT: After eating, he stepped outside.
Please note that, Present participles are NOT "present" and past parciples are not "past." The structure of 4. is Preposition+Gerund:
4. After eating,....
If 'eating' were functioning as part of a verb in 4., it would need a verb to be part of (i.e., is; was).
All the best, :D