I promise ! 除咗呢個字, 仲有乜野字可以用, 令到人哋好鬼相信你呢 ?

Trust me!
Believe me!
I guarantee.
I'm 100% sure.
I beg you trust me, please!!

Guarantee [保證]

E.g: I guarantee this TV will work for 10 years
我保證這台電視會用到十年

Assure [保證. 使放心]
E.g: I assure you that the test result is correct.
我向您保證測驗的結果是正確的

Certain [肯定]
E.g: I am certain that it will be raining tomorrow
我肯定明日會下雨

Sure [肯定]
E.g: I am sure I will be able come back tomorrow.
我可以肯定我明日會能回來

i swear = 我發誓 （常用d .. 對話用)
i vowed never to tell anybody about this =
我發誓永遠都不會把這說出去 .. (小用d ..通常用在書面)