比如He must have a conputer.这句话，因为语气太重，而且可能有理解为“必须”的歧义，可以考虑换个表达方式。比如：
He definitely has a computer.
It is definite that he has a computer.

It is certain that he has a computer.
I am sure that he has a computer.
There is no doubt that he has a computer.
He has a computer for sure.

初三的时候刚开始补英语我就想must有“必须”和“一定”两个意思，怎样才能没有歧义，现在终于知道了。
You must do the laundry.你必须洗衣服
You are doing the laundry for sure.你肯定在洗衣服。
It is certain that he is sleeping.他肯定在睡觉

I am sure that he is sleeping.
我肯定他在睡觉。

He must go to bed now.
He must fall asleep now.
他现在必须去睡觉

It is certain that就比must好多了，最重要的是没有歧义

No one knows for sure.这句话课本上给出的翻译是：没有人确切地知道。

回复：2楼
最后一个句子可能有点问题。

have (got) to也可以表示“一定”。

e.g. This has got to be the most difficult exam I have ever sit.

这一定是我参加过的最难的考试。

今天又学到一个句型：
sb. is guaranteed to do
某人肯定要做某事