What is the difference between as and because? Which one of these sentences is correct?
He stayed home from work as he was sick?
He stayed home form work because he was sick?
Which is correct?
Are they both correct?
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As is used to mean because, but it is also used when two events happen at the same time.
In "I must stop now as I have to go out." it means because, but in "She watched him as the train passed close to his house." it doesn't mean because.
As for the sentences you used as examples, both are correct.
In some contexts, any of as, since, and because can be used. kiamlaluno's example, “I must stop now as I have to go out” is one such context.
However, I believe most native speakers would shun “He stayed home from work as he was sick”, instead preferring either “He stayed home from work since he was sick” or “He stayed home from work because he was sick”.
The major differences between as and because are three, in ascending order of importance:
As has two letters, and because has seven: point to as
As has one syllable, and because has two: point to as
As is often ambiguous, but because never is: 10 points to because
If you care about clarity (I do), then you should never use an ambiguous word when there is a perfectly acceptable unambiguous word that can be used instead.
The two sentences in your question are both grammatically correct and both idiomatic native-speaker versions of the same semantic content. The first is more likely in British English than in American English, to which, however, it is no stranger.
You might want to look at this related SE question and answer: When are "because", "since","for" and "as" interchangeable? And this page of advice about using as and because. And this well-researched Grammar Girl page.
The word "as" is used for expressing comparisons: She was as pretty as an angel; the cold wind was as sharp as a knife. When "as" is used any other way, the writer could be making a much better word choice, such as because, since, while, or when. "As" does not mean because or since. If the dictionary claims it does, it is only doing so because of frequent incorrect usage, not because it is right.
An example above uses the sentence: "She watched him as the train passed close to his house." In this sentence, while or when is a better word choice. Also, I would flip the sentence: When the train passed close to his house, she watched him. Or, While the train passed close to his house, she watched him.