First of all, what is a phrasal verb? Well,
A phrasal verb is a combination of a verb and preposition, a verb and an adverb, or a verb with both an adverb and a preposition.
A phrasal verb has a meaning which is, usually, different from the original verb. That’s what makes them fun, but confusing. You may need to try to guess the meaning from the context, or, failing that, look it up in a dictionary.
The adverb or preposition that follows the verb are sometimes called a particle. The particle changes the meaning of the phrasal verb in idiomatic ways.
There are phrasal verbs that can be separated, some that cannot and others that can be either.
Cheer up! Don’t worry…
Let me cheer you up! It is not that difficult….
As you can see the phrasal verb “to cheer up” can separated.
A phrasal verb is separable when it is transitive, that is, it needs a direct object to express a complete idea. Look at the example:
“Please, turn off the TV” or “Please, turn the TV off”
“To turn off” is a transitive phrasal verb and therefore it is separable. The direct object is the noun “the TV” and that is why both examples are correct. However, if we used the pronoun “it”, we MUST separate the phrasal verb and say:
Please, turn it off.
On the contrary, an intransitive phrasal verb is not separable.
For example, if the boy complains, the mum can say:
When you grow up, you can watch all the TV you want.
“To grow up” is an intransitive phrasal verb and therefore CANNOT be separated.
Here is a good video that explains it better