Conjunctions are words that join different kinds of grammatical structures. They connect words, phrases or sentences and are used to give more information about time, place, persons and things, but also give reasons and express contrast.

TIME when, until, till, after, before, since, while, at first, but then
  Janet was in her room when her mother called.
  He waited until she came.
  After Peter had gone to bed, he read a book.
PLACE where
  He didn't know where he had put his purse.
REASON because, so, therefore / that's why
  He couldn't help them because he didn't have any time.
  She was ill, and so she couldn't go to the party.
  This city is nice to visit, but I wouldn't like to live there.
RELATIVE CLAUSES who (persons), which (animals, things), that
  This is the man who / that won the race.
  Is this the car which / that he bought last summer?


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