Since, as, so that

[Dan]

Hi everybody,

I'd like to know when I use "since" or "as" at the beginning the sentence.

I don't know whether to use "since" or "as".
For example:
Since he was tired, he took a nap.
As he was tires, he took a nap.

Thanks so much.

Dan

[Dan]

Hi all,

I have a questions about so that, since and as.

Can I only use so that in the present sentences, or not?
What about since and as? Can I use only in the past sentences?

Thanks a lot for answering my questions.

Dan

[StrixVaria]

Since he was tired, he took a nap.
Since he is tired, he is taking a nap.

These both work, but they both sound awkward. As sounds even worse (As he was tired, he took a nap.), and so that doesn't really work at all unless you change the meaning. (He took a nap so that he wouldn't be tired anymore.)

You'd be better off using because in this case. It is much more common than since or as, at least in my area (east coast of the USA). These words also very rarely start a sentence in normal speech, as we are actually drilled as children in our basic grammar classes never to start sentences with conjunctions, even though it is possible.