Syntactic tests can clear away such confusion, and a knowledge of such tests is indispensable for anyone studying phrasal verbs.
Syntactic tests for phrasal verbs:
1. Particle movement: particles for transitive phrasal verbs can move either before or after the direct object, and this will determine whether the word in question is a particle or a preposition. For example, “I gave up the keys / I gave the keys up.” The “up” is a particle because it can move. If it were a preposition, “up” could not move: “I walked up the stairs”, but not *I walked the stairs up. As a side note, particle movement is generally not possible with gerunds: “I gave up trying” but not *I gave trying up. Particle movement is also restricted with pronouns: “I helped her out”, not *I helped out her. Particle movement is also unhelpful in analysing intransitive phrasal verbs as there is no complementary noun phrase to facilitate movement.
2. Adverb intervention: Adverbs cannot be placed within the verb phrase, including verb, particle, and object, but must be placed before the verb or at the end: “I help out Sheila often / I help Sheila out often / I often help out Sheila”, but not * I help often out Sheila, I help out often Sheila, I help often her out. Adverbs can, however, be placed between verbs and prepositional phrases: “I went quickly into the room.”
3. Spoken stress: particles are stressed in phrasal verbs, but prepositions are unstressed (unless stressed emphatically in speech). Therefore, one says, “I gave up the keys” (“up” is stressed – particle, transitive phrasal verb) or “the plane touched down” (“down” is stressed – particle, intransitive phrasal verb). A true preposition is unstressed: “I walked up the stairs” (unstressed – preposition, prepositional verb).
4. Translation / synonymy: Phrasal verbs can be translated with a single-unit verb of the same illocutionary force. Therefore, “give up” can be translated as the clearly transitive “relinquish” or “surrender”, while “touch down” can be translated by the clearly intransitive “land”. Translation, however, is not reliable as the sole or even primary method of syntactic testing. Quirk et al. discuss the possibility of translating certain prepositional verbs with single-unit transitive verbs. For example, the sentence “She looked after her son” could be translated “She tended her son” (1155-6). Obviously, “after” is not a particle, as it lacks stress and movement, but this style of analysis, still unresolved in descriptive grammar, confirms the wisdom of using other tests when checking for phrasal verbs. Phrasal-prepositional verbs are also difficult to analyse by this means alone because of the possibility to translate them with single-unit transitive verbs.
5. Passivization: Transitive phrasal verbs can be rendered in the passive for two reasons: because they are transitive and have the capacity for the inversion of logical subjects and objects, and because doing so does not violate the syntactic frame of a prepositional phrase. Therefore, the sentence, “I gave up the keys” can be rendered in the passive: “The keys were given up by me.” However, a prepositional verb at least prescriptively resists rendering in the passive: “I walked up the stairs” would not traditionally be rendered thus in the passive: “The stairs were walked up by me”, even though “to walk up” could be translated with the transitive verb “to ascend”, which could easily be rendered in the passive. However, as Denison discusses at length, and as Quirk et al. point out (1156-7), prepositional verbs have been rendered increasingly in the passive. Therefore, passivization is also by no means a stand-alone syntactic test of phrasal verbs.