ensure確定，確保：
1.The book ensured his success.那本書確定了他的地位
2.These pills could ensure you a good night\'s sleep.這些藥丸可 以確保妳睡壹宿好覺。
3.ensure product quality 確保產品質量
guarantee保證，擔保,承諾對某事負法律責任：
1.I can guarantee it\'s true. 我保證那是真的。
2.guarantee to pay debts. 保證還債。
assure 明確或有信心的告訴某人，保證某事：
1.I assure you he will be safe with us. 我保證他和我們在壹起
會完全的。
2.assure sb/oneself of sth. 使某人自己對某物確信不疑。
She was able to assure herself that nothing had been taken from her purse.她確信錢包裏什麽東西都沒有被拿去。
certify強調書面證明某事物。
warrant證明某物正當有理，保證某物為真品正品:
This material is warranted to be pure silk.這種料子保證是純絲的。