Prepositions often come at the ends of clauses in English. This happens in several kinds of structure:

a) questions beginning what, who, where etc.

• What are you looking at?
• Who did you go with?
• Where did you buy it from?

b)    relative clauses

• There's the house (that) I told you about.
• You remember the boy I was going out with?

с)    passive structures

• I hate being laughed at.
  
• They took him to hospital yesterday and he's already been operated on.
  

d)     infinitive structures

• It's a boring place to live in.
• I need something to write with
  

In a more formal style, we can put a preposition before a question-word or a relative pronoun.

• To whom is that letter addressed?
  
• She met a man with whom she had been friendly years before.
  
• On which flight is the general travelling?