Adverb particles and prepositions

Words like down, in, up are not always prepositions. Compare:
- I ran down the road. Please sit down.
- He's in his office. You can go in.
- Something's climbing up my leg. She's not up yet.
In the expressions down the road, in his office and up my leg, the words down, in and up are prepositions: they have objects (the road, his office and my leg).
In sit down, go in and She's not up, the words down, in and up have no objects. They are adverbs, not prepositions.

• Small adverbs like these are usually called 'adverb(ial) particles'. They include above, about, across, ahead, along, (a)round, aside, away, back, before, behind, below, by, down, forward, in, home, near, off, on, out, over, past, through, under, up. Many words of this kind can be used as both adverb particles and prepositions, but there are some exceptions: for example: back, away (only adverb particles) ; from, during (only prepositions).
  
• Phrasal verbs
  Adverb particles often join together with verbs to make two-word verbs, sometimes with completely new meanings (e.g. break down, put off, work out, give up). These are often called 'phrasal verbs'.
  Note that, unlike most other adverbs, adverb particles can come just before the object of a verb. Could you switch off the light?
  
• Adverb particles are often used, rather like adjectives, as complements of the verb be.
  Why are all the lights on?
  Hello! You're back!
  The game will be over by 4.30.