you should careful when crossed the street in the big cicy我这个crossed用的对吗

You should be careful when cross the streets in the big city.
原句有几处错了.用cross其实是省略了when 后面的you

You should BE careful when CROSSING the street in the big city.
嗯，应该这样用。

不对
问题很多
you should BE careful when CROSSING the street in a big city( the 也可以，如果有上文的话）
careful 是形容词，要用be动词修饰
when是介词，介词后面不加动词，因此就将动词变成动名词（ing形式）

要用crossing `~~~~~!

动词是对的,但不需要用过去式,用一般现在时就可以.另外,前面的半句是错的,正确如下：
You should BE careful when you cross the street in the big city.

不对.
cross 即可.
还有should be careful.