You can use because, as, since, or for to give an explanation for something or to explain why you say something.

Because is the most commonly used word for giving a reason or motive for something. If someone asks a question beginning with Why?, you can reply using because.

1. I said I wouldn’t go. — Why? — Because I didn’t want to leave you.
2. Why shouldn’t I come? — Because you’re too busy.

If you have said that something is the case and you want to say why it is the case, you usually add a reason clause beginning with because.

3. She finally walked off and caught the bus, because she could not really afford to pay both Eileen and a taxi.
4. I couldn’t see Helen’s expression, because her head was turned.

You can also use as or since to introduce a reason clause, especially in writing.

5. A brief note on The National Portrait Gallery’s character is necessary, as the word ‘Gallery’ can be misleading.
6. I realized it wouldn’t be in my interests to deceive him since I planned to deal with his bank for many years.

The reason clause is sometimes put first.

7. As they have been on the winning side, they may have temptations to extend their borders.
8. Since evaporated milk is about twice the strength of fresh milk, you always dilute it with at least an equal amount of water.

If you want to say that there is a special reason for something, you can use words like ‘especially’ or ‘particularly’ in front of as or since. When you do this, you put the reason clause after the main clause.

9. I was frightened when I went to bed, especially as my room was so far up.
10. It was nice to have someone to talk to, particularly as it looked as if I was going to be there all night.

In stories, for is used to explain or justify something that has just been said. This is an old-fashioned use.

11. After a while he seems to feel the need of company again, for he suddenly scrambles back into the kitchen.
12. The diet we gave her seemed satisfactory, for she grew well.