1. They could( couldn't) help you. They were not here

=>....They could not help you because they weren't here.......

=>.......They weren't here so they couldn't help you.......

2. I can't go out. It's raining

=>.....I can't go out because it's raining......

=>......It's raining so I can't go out......

3. We haven't got any food. We are going shopping

=>.......We haven't got any food so we are going shopping........

=>.....We are going shopping because we haven't got any food.....

4. There was fog. Their flight was delayed

=>....There was fog so their flight was delayed.......

=>.....Their flight was delayed because there was fog.......

5. The film was boring. We decided to go home

=>......The film was boring so we decided to go home.........

=>.......We decided to go home because the film was boring.......

6. I couldn't do this exercise. I asked Tom for help

=>.....I couldn't do this exercise so I asked Tom for help........

=>.....I asked Tom for help because I couldn't do this exercise....