assure, ensure, insure, guarantee都含有一定的"保证"之意

assure 表示向某人保证某事一定会发生(内含有让某人放心之意，以消除疑虑) assure sb. of sth /assure sb. that…

He assured us of his ability to solve the problem．他向我们保证他有能力解决这个问题。

The witness assured the judge that he was telling the truth .

ensure “确定,保证” 确保某种行为一定会发生 ensure sth. /ensure that…

I will ensure that the car arrives by six o’clock.

我保证汽车六点钟以前到。

This medicine will ensure you a good night’s sleep.

这药能保证你睡一夜好觉。

insure 意思是为防不测向保险公司付钱投保

My house is insured against fire．我的房子保了火险。

guarantee 对某事物的品质或人的行为"提出担保",对事物、商品等无质量,性能等方面的欺诈行为做出保证

The cooker is guaranteed for five years. 本炊具保用五年。

Can you guarantee me a job when I get there?

你可以确保我到那儿就可以找到一份工作吗?