实用口语：cross my heart

Cross my heart./cross your heart. 我保证/你保证.

e.g. Promise you will come for us. Cross my heart.  你保证你会来找我们. 我保证.

e.g. I’ll be right back, okay? Promise? Cross my heart. 我很快就会回来. 你保证? 我保证. 

e.g. I promise -- 100%. Cross my heart. 我100%保证.

e.g. You have to swear you will not tell anyone! Cross your heart! Do it.  你发誓你不跟人任何人. 你保证.