Because, since, as

Unlike Japanese, English doesn't really segment words into formal or informal. "Because," "since," and "as" can be used in both formal and informal situations.

I can think of a few ways to use these words off the top of my head. I'm sure that there are other ways of using them.

Because (giving a reason):

1. I was late because the train broke down.
2. The class was cancelled because of the typhoon.

Since (giving a reason):

1. He was scolded by his boss since he has always been late for meetings.
2. Since it is such a lovely weather, I decided to take a walk around the park.

Since (time):

1. He has not been happy since he was married to her.
2. She has been in the national basketball team since she was in high school.

As (reason):

1. As there was a thunderstorm, I decided not to leave the house.
2. You should not stare into other people's eyes as they might be offended.

As (some other ways of using):

1. The dog suddenly appeared as he was about to leave the house.
2. You are as lazy as a pig.

Hope this helps.