Question

What word is being modified by the underlined phrase in the following sentence? Is the jacket on the couch yours or mine? On the couch is underlined.

Answer 1

Is the jacket on the couch yours or mine?

The word modified by on the couch is jacket.

On the couch is a prepositional phrase. A prepositional phrase is a phrase that  starts with a preposition, in this case ON, and shows where an object is located, COUCH. 

Where is the jacket? ON THE COUCH.

Prepositional phrase is composed of a preposition plus an optional modifier plus a noun or a pronoun or a gerund.