A phrasal verb is one that’s followed by an adverb or a preposition, and together they behave as a semantic unit. (The adverb or preposition following the verb is called a particle.) A phrasal verb functions the same way as a simple verb, but its meaning is idiomatic:
The numbers don’t add up.
That’s an offer he can’t turn down.
Call off the wedding.
Phrasal verbs are among the most difficult concepts for ESL students to grasp; the particle changes the verb in a way that’s entirely colloquial.
Some phrasal verbs are separable: their particles can be separated from the verb and a noun inserted. Others cannot be separated.
She added up the numbers.
She added the numbers up.
We have enough to fall back on.
He broke into the conversation.
Some are both separable and inseparable, depending on their meaning.
She threw the ball up.
She was so nauseated, she felt like throwing up.
One of the biggest difficulties with phrasal verbs is that there’s no guideline for which ones are separable and which are not. Native English speakers grow up incorporating phrasal verbs into their daily conversation and know how to form them intuitively. Unfortunately, non-native speakers must rely solely on memorization.