Some words that look like prepositions do not always function as prepositions. Consider the following sentences:
The balloon rose off the ground.
The plane took off without difficulty.
In the first sentence, off is a preposition that introduces the prepositional phrase off the ground. In the second sentence, off neither functions as a preposition nor introduces a prepositional phrase. Instead, it combines with took to form a two-word verb with its own meaning. This kind of verb is called a phrasal verb, and the word off, when used in this way, is called an adverbial particle. Many prepositions can function as particles to form phrasal verbs.
The verb + particle combination that makes up a phrasal verb is a single entity that cannot usually be torn apart.
Exceptions include some phrasal verbs that are transitive, meaning that they take a direct object. Some of these verbs have particles that may be separated from the verb by the object.
I picked up my baggage at the terminal.
I picked my baggage up at the terminal.
If a personal pronoun is used as the direct object, it must separate the verb from its particle.
In some idiomatic two-word verbs, the second word is a preposition. With such verbs, the preposition can never be separated from the verb.
We ran into our neighbor on the train. [not ran our neighbor into]
The combination run + into has a special meaning (find by chance). Therefore, run into is a two-word verb.