1. Somebody saw her cross the street. - She was seen to cross the street.
2. Somebody saw her crossing the street. - She was seen crossing the street.
In #1, we are saying that she started on one side of the street and finished on the other. In #2, we are simply saying that she was observed in the act of moving across the street; it is possible that she was run over bbefore she reached the other side,
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