ensure : 侧重使人相信某个行为或力量产生的结果。
- The airline is taking steps to ensure safety on its aircraft.
- The role of the police is to ensure (that) the law is obeyed.
- Their 2–0 victory today has ensured the Italian team a place in the Cup Final/ensured a place in the Cup Final for the Italian team.
- The house is insured for two million dollars.
- All our household goods are insured against accidental damage.
- I'm not insured to drive his car.
assure : 侧重指消除某人思想上的怀疑或担心，从而有达到目的的保证感，但不如ensure普通。
- The unions assured the new owners of the workers' loyalty to the company.
- "Don't worry, your car will be ready tomorrow," the mechanic assured him.
- She assured him (that) the car would be ready the next day.
- The prime minister assured the electorate (that) taxes would not be increased after the election.
- You can rest assured (= feel confident) that I shall be there as promised.
guarantee : 指对事物的品质或人的行为及履行义务、义务等承担责任的保证。
- European Airlines guarantees its customers top-quality service.
- The label on this bread says it is guaranteed free of/from preservatives (= it contains no preservatives).
pledge : 正式用词，指通过郑重许诺、协议或立誓等保证承担某一义务或遵守某一原则。
- We are asking people to pledge their support for our campaign.
- If you join the armed forces, you have to pledge allegiance to your country.
- So far, £50,000 has been pledged (= people have promised to pay this amount) in response to the appeal.
- Both sides have pledged to end the fighting.
- I've been pledged to secrecy.
promise : 侧重表自己的主观意向，设法用语言使人感到稳当可靠。
- He promised faithfully to call me every week.
- The government have promised that they'll reduce taxes.
- Promise me (that) you won't tell him.
- I'll look for some while I'm at the shops but I'm not promising anything.