HOW TO EXPRESS A COMPLEX IDEA BY CONNECTING TWO MORE SIMPLE ONES

We can join two or more simple sentences to express a complex idea which relates them. The resulting sentence is called a compound or complex sentence. A compound sentence is made up by two or more simple sentences joined together by connectors
 
          I    love    you    because    you    are    so    sweet
         ( main clause  + connector + subordinated clause )
 
 
Very often, we can revert the normal order of the sentence placing the subordinated clause first. In this case, we should put a comma separating both sentences (when speaking, the intonation breaks too)
 
          She wasn't there when I phoned
                  (m.c.          +  c.  +     s.c.)
 
          When I phoned , she wasn't there
              (s.c.        comma          m.c.)
 
 
NOTE: a few connectors, however, can't go before the main clause, especially: AND, BUT, SO.
        I phoned her but nobody answered   (not: *But nobody answered, I phoned her)
        I went there and said hello       (not: *And said hello, I went there)

KINDS OF CONNECTORS
 
There are two kinds of connectors: conjunctions and free connectors
 
CONJUNCTIONS are part of the subordinated clause. They take two simple sentences and make one complex sentence:
 
    She fell down after you turned off the light
    I like it although it's very expensive
    He was reading while she was watching TV
 
FREE CONNECTORS also connect two different sentences but they are not part of any of them. They connect two simple sentences but don't join them. Grammatically speaking, both sentences stay simple, but the relationship between them is the same as in a compound sentence. Free connectors are separated from both sentences by commas or a stop and a comma
 
     I turned off the light. Afterwards, she fell down
     I like it. Nevertheless, it's very expensive
     He was reading. Meanwhile, she was watching TV

Compare:

I went to bar before she phoned me = I went to the bar. Later, she phoned me 

In that example BEFORE is a conjunction and LATER is a free connector. Same meaning, different grammar.

In a normal conversation we don't normally use free connectors (but there are a few which are common when speaking). Most free connectors are too elegant, beautiful and formal for normal speech, but they are an excellent choice for writing. So if you are writing a formal composition, do not forget to use some free connectors and your writing will sound much more elegant. But careful, if you use too many free connectors your composition may sound too pedantic and overloaded, so use conjunctions and some free connectors here and there.
 
Some prepositons can express the same idea as a connector, but prepositions work inside just one simple sentence (only one verb):
 
      I'm late because there was a lot of traffic   (connector, two sentences)
      I'm late because of the traffic   (preposition, one sentence)

NOTE: when we use a verb, prepositions and conjunctions are always followed by -ing :
                         He slept in my house because sleeping outside was too cold
                         She cried after falling down
                         I'm thinking about buying a new car