I read one of these words today where it seemed to me that the other was more appropriate.

Problem:
Most dictionaries treat these two words as synonymous conjunctions, but one is preferable over the other for expressing cause and effect.

Explanation:
Many, if not all, dictionaries indicate that the second or third definition of the word “since” when used as a conjunction is “because”.

An unambiguous example of using “since” as a synonym for “because” is “I kissed her since I love her.”

However, it is easy to construct an ambiguous sentence with the conjunction “since”.

For example, “I decided to learn French since my company moved me to Paris.” has two possible meanings:

1. A causal meaning — “I decided to learn French because my company moved me to Paris.”;
2. A temporal meaning — “I decided to learn French after my company moved me to Paris.”

The temporal version clearly tells us when the writer decided to learn French. The causal version tells us why the writer decided to learn French.

A commenter at another website noted that Bryan Garner wrote in his book Garner’s Modern American Usage that the causal meaning of “since” has existed for more than one thousand years.

I believe that the use of “since” as a substitute for the conjunction “because” is consistent with my “Devolution toward Simpler” linguistic hypothesis. It is simpler to say or write the one-syllable, five-letter “since” than it is to say or write the two-syllable, seven-letter “because”, and clarity can be easily sacrificed for simplicity.

Solution:
To avoid confusion, prefer the conjunction “because” over the conjunction “since” when joining two sentences in a causal relationship. And prefer the conjunction “after” over the conjunction “since” when joining two sentences in a temporal relationship, unless “since” clearly carries a temporal connotation.