- Pick up the pen.
- Pick the pen up.
- Pick it up.
- *Pick up it. (ungrammatical)
The idiomatic string pick up is often described as a separable phrasal verb (I don't believe in phrasal verbs so I don't use this term). As shown in examples (1-4) the Direct Object of the verb pick can go either before or after the word up—unless it is a pronoun, in which case it must go before it.
Because the word up can appear without a following noun phrase in such idioms, it has often been regarded an adverb here. In most (but not all) modern grammars, however, words like up are classified as prepositions, whether or not they take a noun phrase as a Complement (in the same way that verbs are still regarded as verbs whether or not they take a Direct Object). In grammars such as Oxford Modern English Grammar by Bas Aarts (2011), for example, the word up would be classified as a preposition here.
All the evidence indeed seems to show that words like up in this example are prepositions and not adverbs. For example, up can be modified by the specialised adverb right which modifies prepositions, but not adverbs:
- He pick it right up, and ran out of the door.
Unlike adverbs, which are usually modifiable by the word very, the word up cannot be modified by very (and note that up-ness has degrees, it is semantically gradable):
- *He picked it very up, and ran out of the door. (ungrammatical)
The verb pick, of course, remains a verb—whether or not it is used in idiomatic verb-preposition combinations.
Bert Capelle classifies words like up here as a completely different part of speech, for which he uses the term particle. Note that this is not how other writers use the word. He does not argue that such words are adverbs.