I believe you're looking for the word the prepositional phrase modifies.
1. "in 1978" modifies "invention." Does that make the phrase an adjective or an adverb?
What do you think about the second sentence?
What is the modifier of the prepostional phase and what kind of prepostional phrase is it? Adverb or Adjective?
1. Its invention in 1978 revolutionized technology.
2. The microwave oven was invented in 1946.
3 answers
Is "in 1978" and adjective phrase then?
Yes. "In 1978" is an adjective phrase.
1 answer