She is never completely sincere in what she says about people.
她談論別人，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。