As, because, and since overlap in meaning. This can lead to confusion, because since and as have alternative definitions. Since also means "from a past time up to the present", and as has many (at least 32!) alternative definitions, including:

1. to the same degree, amount, or extent; similarly; equally
2. for example; for instance
3. thought to be or considered to be
4. in the manner of
5. at the same time that

Because is the only unambiguous one of the three. It means "for the reason that" or "by reason of". (Never use the redundant phrase "the reason is because".)

Consider--

A. I didn't hear her enter as I was concentrating.
B. Since the instrument broke, I've been very busy.

Does A mean that because I was concentrating, I didn't hear her enter, OR does it mean that she entered at the same time that I was concentrating? Similarly, for B, have I been very busy because the instrument broke, OR busy just in the time after the instrument broke until now? My suggestion is to use as sparingly in your texts, solely for variety from because and since. Look for ambiguities, particularly in time.

Try finding the least ambiguous answer to put into the sentences below.

1. As (= in the manner of) in the kitchen, we are now going to cook up a sample.
2. Since the first of the year, I was in America four times.
3. Because you left the door of the refrigerator open, we will lose all our food.
4. As (= to the same degree) fat as the Titanic, he couldn't fit through the door.

For sentence 4, you could also write "Because he was as fat as the Titanic, he couln't fit through the door." Here, the emphasis lies on the casual relationship between being overweight and movement. The intention of 4, however, was to invoke imagery and relate an event. "He was as fat as the Titanic. He couldn't fit through the door." The construction of the sentence gives a strong clue that as is the best answer.

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Created June 3, 1999, by Nancy Burnham and Fred Hutson.