Let's compare participial phrases (as we just discussed) with reduced adjective clauses.

Are they the same or different?

In many common cases, they are essentially the same thing in their final form and function.

• A "reduced adjective clause" describes the origin or the process of shortening a full adjective clause (also called a relative clause).
• A "participial phrase" (when used adjectivally) describes the resulting grammatical structure after that reduction has occurred (or a similar structure formed directly).

Think of it this way: Many adjective clauses can be "reduced," and when they are, they become participial phrases.

How the Reduction Works:

An adjective clause modifies a noun or pronoun and usually starts with a relative pronoun (who, whom, which, that) and contains a subject and a verb.1

1. Reduction leading to a Present Participial Phrase (Active Voice):
   • If the adjective clause contains an active verb, and the relative pronoun is the subject of that verb, you can often reduce it by removing the relative pronoun and changing the verb to its present participle (ing) form. If the verb is a form of "be" + present participle, you just remove the relative pronoun and the "be" verb.2
   • Full Clause: The man who is talking to John is my brother.
   • Reduced: The man talking to John is my brother. (talking to John is a present participial phrase).
   • Full Clause: The dog that was wagging its tail greeted its owner.
   • Reduced: The dog wagging its tail greeted its owner. (wagging its tail is a present participial phrase).
2. Reduction leading to a Past Participial Phrase (Passive Voice):
   • If the adjective clause contains a passive verb (be + past participle), you can often reduce it by removing the relative pronoun and the "be" verb, leaving just the past participle to start the phrase.
   • Full Clause: The letter which was written by Sarah arrived today.
   • Reduced: The letter written by Sarah arrived today. (written by Sarah is a past participial phrase).
   • Full Clause: The table that is made of solid oak is expensive.
   • Reduced: The table made of solid oak is expensive. (made of solid oak is a past participial phrase).

Conclusion:

• The participial phrases we discussed in the previous answer (like talking to John, written by Sarah) are precisely what you get when you reduce certain types of adjective clauses.
• So, functionally, a participial phrase acting as an adjective is often a reduced adjective clause.
• Grammarians might use "reduced adjective clause" to emphasize the relationship to the full clause and the transformation process. They might use "participial phrase" to describe the structure itself, regardless of whether it's thought of as a reduction.

For practical purposes, especially when teaching, understanding that these participial phrases function like adjective clauses and can often be derived from them by removing the relative pronoun and be verb is a very useful concept. They are two ways of looking at largely the same grammatical structure and function.