Question

一道初二英语题！求高人

文章（包括我填对的格子就一并打出来了）
You should have good study habits.Then you can spend less time but learn more.
First,make full use of your time.Are your books open?If so,please turn off your TV and put away your snacks.Of course,you need some time to play.But you should play after study.Remember:Study comes first.
Second,listen to your teachers carefully in your class.You know,it is very important.If you have questions,just ask your teachers.They are very glad to help you.
Third,do your homework by yourself.If you can't work out a problem successfully,you can ask for help.But you can't just copy.Make sure that you u_______ it.
Finally,take a note with you.Sometimes you may have some ideas or questions.Please write them down.Then you can talk to your teachers,parents or friends.
These four tips are helpful to you.Please have a try,my boy!You will succeed if you have good study habits.
画线的地方肯定是填understand没有问题，但是是用什么时态呢？我写的是过去式understood但有同学写的是原形，纠结了。
不是应该确保（说话时已经）懂了吗？
高人求解答！

Answer 1

这里的that引导make sure的从句
一般来说，这种从句与make的时态是保持一致的。这里的make是一般现在时，所以从句里的谓语动词understand也用一般现在时。
就make sure 这个词而言，当它用来引导一句虚拟语气的句子，后面的谓语动词就要据情况而定了。

Answer 2

应该用understand

原因是

1. 这是一个一般现在是，

2. 前文所有的都用了原形，你怎么能用过去是呢

   希望你能真正明白

如果不懂可以反问哦

问到会为止

Answer 3

make sure you have understood it. 或make sure you understand it.
确保你已经明白它了。不是过去时，而是完成时或一般现在时。

Answer 4

不是这样的 全文用的都是现在时

Answer 5

make sure that 一般跟一般现在时，

Answer 6

understand