简单的英语时态题

简单的英语时态题
1 Beforeyou leave the hotel ,make sure to check if there's anything you ___behind.
A leave B left C have left Dare leaving
选C
2 Before you leave the hotel ,make sure to check if there's anthing __behind
选项同上,选B
3He was so absent-minded,he could hardly keep his mind on what she__
A said B had said C had been saying D was saying
选D
4You are late again
I'm sorry ,i __in the heavy snow for some time before i got it out
A stuck Bgot stuckCwas sticking D had stuck
选b
5When faced with difficulty,he sometimes ___to us,though not very often.
A turned Bturns C has turned Dwould turn
选b
这几个题目我都选对了,几乎都是语感,对时态的概念不是很清楚所以经常会做错.比如第一题,为什么要选完成时.
如果有时间,可不可以帮我详细的讲讲这几道时态题目,不要只说对的选项,解释一下其他的选项为什么不对,至少让我明白点什么（我对这东西十分头痛）
不胜感谢.
先谢谢回答
三楼的那个第一题，“有没有东西落下”是在我“离开饭店前做的”我现在不是还没离开饭店嘛，为什么要说是过去的过去呢？这里是不是强调一个结果什么的
那个第四题，我刚刚怎么样，为什么要用进行事态呢？反正都已经是过去的事情了。
那个第三题不能就因为一个was就确定是她正在说啊。其他的怎么不行了。