I feel constrained to write and ask for your forgiveness.

我不得不写信请你原谅.

《现代英汉综合大词典》

I feel constrained to write and ask you to forgive me.

我不得不写信给你请求原谅.

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I feel constrained to write and complain in the strongest possible terms.

我深感责无旁贷,必须笔诛墨伐.

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