"Because", "since", and "as" can be used interchangeably in some cases, because they can all be used for the same purpose: to answer the question, "Why?". In other words, the sentence will make sense, regardless of which word you use, although it may not sound the most natural. For example: 1. I didn't go for a walk today because it was raining. 2. I didn't go for a walk today as it was raining. 3. I didn't go for a walk today, since it was raining. Most people would probably use "because". You don't see "as" used as much this way nowadays, although I use it rather frequently in my writing. If you ask me, "since it was raining" sounds awkward at the end of a sentence, like it's hanging off. "As" and "since" are usually used at the beginning of the sentence, but this is definitely not a rule. Here is a blog that gives more details on the usage of these words: http://gmattoughies.blogspot.com/2009/07/because-vs-as-vs-since.html I hope that helped. :)

'Because', 'As', and 'Since' are very similar yet different words. 'Because' usually explains an idea (for example, "She is tired because she worked all day"). 'Since' is used typically for explanation of time after an event (for example, "Since she left, he has been sad"). 'As' is used often with comparisons of things, (for example, "He is as strong as a elephant.") Does this help?