At the minimum, a prepositional phrase will begin with a preposition and end with a noun, pronoun, gerund, or clause – the “object” of the preposition.
There isn’t a prepositional phrase as there’s no preposition in your sentence. “Suddenly” is a adverb, which you can see more clearly if you rewrite your sentence, “The battery tester’s tiny red lights started to flicker suddenly. “Suddenly” describes the verb “to flicker” so is an adverb.
Have a look at:
Main Entry: flicker
Part of Speech: verb
Definition: sparkle, flutter
At the minimum, a prepositional phrase will begin with a preposition and end with a noun, pronoun, gerund, or clause – the “object” of the preposition.
There isn’t a prepositional phrase as there’s no preposition in your sentence. “Suddenly” is a adverb, which you can see more clearly if you rewrite your sentence, “The battery tester’s tiny red lights started to flicker suddenly. “Suddenly” describes the verb “to flicker” so is an adverb.
Have a look at:
Main Entry: flicker
Part of Speech: verb
Definition: sparkle, flutter
“to flicker”
this page has a complete list of prepositions