A prepositional verb is a verb that forms a combination with a preposition without yielding a different meaning other than what is obvious.

If you read the definition between the lines, you will deduce that it is possible that, in some instances, the combination of a verb with a preposition will yield another meaning.

You are right! When this happens, we cannot say we have a prepositional verb but a Phrasal Verb. In the case of Phrasal Verbs, their meanings cannot be arrived at by the individual meanings of the verbs and prepositions put together.

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https://akademia.com.ng/prepositional-verbs-definition-and-examples/