As to my understanding, you must use "一个something" when using 过了 in reference to something in the past, but is it possible to attach 以前的 before something in the past so that it clarifies just as 过了 does?
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no, sth + 过了 which means past the position of sth, mainly, sth+过了 modifies the position, not time.
the only one case when 过了 modifies time is 过了很久,that BeckyGuo has metioned before.
过了很久+ sentence,which is talk about what is happened now.
以前的 and 过了, usually they used in different way.
Example:
你急不急的过了一个路口我们找到那个咖啡厅?
(Do you remember that street turn-off where we found that coffee shop?)
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here, there are some mistake in the sentence, we rewrite it firstly.
你急不急的过了一个路口我们找到那个咖啡厅? SHOULD BE 你记不记得(我们找到那个咖啡厅的)那个路口。
we don't put the subordinate clause after the sentence.
on the other hand, if you want to say at the corner, you can't use 过了,过了一个路口means past a intersection。
if you want to use 以前, 你记不记得以前我们找到那个咖啡厅的那个路口。this sentence means "Do you remember the street corner that we found out the coffee shop before"
How do you use 以前 as a replacer for 过了 as a past-indicator....or can you? I've heard that 以前 can indicate a change has taken place since the past...but I'm not sure if that is something implied?
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So actually, they can not directly replace.