Some words that look like prepositions do not always function as prepositions. Consider the following two 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 does not function as a preposition. Instead, it combines with took to form a two-word verb with its own meaning. Such a 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 usually cannot be torn apart.

The exceptions are the many phrasal verbs that are transitive, meaning that they take a direct object (31k). Some transitive phrasal 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 (such as it, her, or him) is used as the direct object, it must separate the verb from its particle.

I picked it up at the terminal.

Some idiomatic two-word verbs, however, are not phrasal verbs.

We ran into our neighbor on the train.

In such verbs, the second word is a preposition, which cannot be separated from the verb. For example, a native speaker would not say We ran our neighbor into on the train. Verbs like run into are called prepositional verbs, which are another kind of two-word verb.

In the preceding sample sentence, ran into consists of the verb ran followed by the preposition into, which introduces the prepositional phrase into our neighbor. Yet to run into our neighbor is different from a normal verb + prepositional phrase, such as to run into the room. If you know the typical meanings of run and into, you can interpret to run into the room. Not so with to run into our neighbor; the combination run + into has a special meaning (“find by chance”) that could not be determined from the typical meanings of run and into.

Prepositional verbs include such idiomatic two-word verbs as take after, meaning “resemble” (usually a parent or other older relative); get over, meaning “recover from”; and count on, meaning “trust.” They also include verb + preposition combinations in which the meaning is predictable, but the specific preposition that is required is less predictable and must be learned together with the verb (for example, depend on, look at, listen to, approve of). There are also phrasal-prepositional verbs, which are verb + adverbial particle + preposition sequences (for example, put up with, look forward to, give up on, get away with).