Because / Since / As / For

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Because

We use because to give the reason of something that is important for the listener.

Because my brother didn’t keep his promise, I couldn’t go fishing.
Because I woke up late, we missed the bus.

Note:

When we want to emphasize the reason, we use the because clause at the end.

Why are you crying? I am crying because we lost the game.
We do not say as / since we lost the game.

I will punish you because you lie to me. (the reason is stressed)

As and Since

Like because, we use as and since to give reason but with as and since, the reason should already be known by the listener.

As we lost the final game, we couldn’t qualify for the semi-final.
(that we lost the final game is not something new to the listener.)

Since I was hungry, I wanted to eat something first.

For

For also introduces a new reason to the listener as because does. For-clause cannot be used in the beginning.

I had to stay at home and finish my assignment – for the deadline was soon.
I am going to Los Angeles for the NBA finals.