Since has a couple of meanings. The most common use is to mean "from a time in the past, until a more recent moment." Like in the sentence: I have been waiting here since 6:00. This means the person has been waiting from 6:00 until the present moment. Since can also mean "because" like in the sentence: Since I lost my keys, I can't drive. This means that because the person lost their keys, they cannot drive.

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