She is never completely sincere in what she says about people.
她谈论别人，一向不尽坦诚。
She was sincere and genuine in her wish to make amends for the past.
她诚恳而真心地希望弥补过去的错误。
I am writing to express my sincere gratitude for your help in my study.
我写这封信是为了感谢你在学习上对我的帮助。