Hey Daniel, that's a great question! Here's a clue: If there's a second action in the sentence, "As" almost always means "While" or "At the time of". For example: "As [something happened], [something else happened]" pretty much always means "while". "My friend arrived just as we were finishing ordering dinner" "The doorbell rang as soon as I sat down" "As the symphony began, Michael felt a strange emotion"

Ryan's answer is good. Generally, ask yourself what makes more sense: AT THE TIME or BECAUSE. Sometimes it's impossible to tell: "I told him to take my chair, as I was leaving" Actually, now that I think about it, the punctuation may help: Take my chair, as I was leaving sounds like BECAUSE I was leaving Take my chair as I was leaving sounds like AT THE TIME I was leaving