Many verbs in English are followed by an adverb or a preposition (also called a particle), and these two-part verbs, also called phrasal verbs, are different from verbs with helpers. The particle that follows the verb changes the meaning of the phrasal verb in idiomatic ways:

• drop off - decline gradually

  The hill dropped off near the river

• drop off(2) - fall asleep

  While doing his homework, he dropped off.

• drop off(3) - stop and give something to someone

  Would you drop this off at the post office?

• drop out - cease to participate

  After two laps, the runner dropped out.

Some particles can be separated from the verb so that a noun or pronoun can be inserted, and some particles can't be separated from the verb. In addition, some phrases are intransitive, meaning they cannot take a direct object.

• Separable
  add up (meaning: to add)

  Correct: She added up the total on her calculator.
  Correct: She added it up on her calculator.

• Inseparable
  get around (meaning: to evade)

  Correct: She always gets around the rules.
  Incorrect: She always gets the rules around (This construction makes no sense in English.)

• Intransitive
  catch on (meaning: to understand)

  Correct: After I explained the math problem, she began to catch on.
  Incorrect: She began to catch on the math problem. (catch on cannot take a direct object in this meaning.)
  Correct: She began to catch on to the math problem. (the word to makes the math problem an indirect object, which is acceptable in this meaning.)

Unfortunately, there is usually no indicator whether an idiomatic phrase is separable, inseparable, or intransitive. In most cases the phrases must simply be memorized. Below is a partial list of each kind of phrase.