She is never completely sincere in what she says about people.

　　她談?wù)搫e人，一向不盡坦誠。

　　She was sincere and genuine in her wish to make amends for the past.

　　她誠懇而真心地希望彌補過去的錯誤。

　　I am writing to express my sincere gratitude for your help in my study.

　　我寫這封信是為了感謝你在學習上對我的幫助。