Try to decide how long it has been since you were sure of your position.
这是《英语阅读丛书》第一册上的一句话。我觉得 since you were sure of your position. （自从你确知你的位置以来，在你确定你的位置以后）似乎与句意不符。
这种用法对于学习英语的中国人来说,似乎有点难于理解。但是,如能抓住它的特点,还是可以掌握的。
当 since 从句的动词为 was (were) 时, 由于 be 是个持续性动词, 与它所联系的常常是一个时间段, 而不是一个时间点。但是, 如大家所知道的, since 从句有时候是用来表示主句动作的起点的 (from that point of time)。那么主句动作的起点是从 since 从句所表达的时间段的开始点算起呢? 还是从该时间段的结束点(终点)算起呢? 这就给我们的理解和翻译带来了困难。请考察下面的例句:
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I have lived here since I was a child.
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Since he was in the army, he has been in much better health.
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I haven't seen him since we were boys together.
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It's been ten years since they were last here.
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We had not met since we had been at college together. (Wilde)
看来,我们只能根据句意来加以判断。句(1)和句(2)显然是从 since 从句的开始点算起的。因此应分别译为:
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从我是个孩子的时候起就一直住在这里。
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打他参军后身体健康多了。
但是句(3)、(4)、(5)却是从 since 从句所表达的时间段的结束点算起,因此应译为:
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自从我们孩童时分开以来我一直没有见到他。
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自从他们上次离开这儿已经过了10年了。
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自从我们大学毕业后,我们再没见过面。
你所提问的《英语阅读丛书》上的句子与句(3)很相似:主句的动作起点应当从 since 从句所表达的时间段的结束点算起。
尽量判定出在你不能确定自己位置以后过了多长时间。
(参见 since he was in the army)