To show why something happened, what we wanted to get as a result.

To show the reason for something – the reason will be in the 1st part of the sentence.

To show the contrast between two things / opinions / events / etc.

To show the reason for something – the reason will be in the 2nd part of the sentence.

I ordered all the Christmas presents in October _______ I don’t need to stand in a queue for them in December.

I love learning something new, _______ I decided to download a language learning app.

I didn’t like this subject at first, _______ then it became my favourite.

I’m sorry, I didn’t do my homework ________ I wasn’t feeling well.