请以书面形式确认您的预订。

Please confirm your reservation ___in ___writing.

我以书面形式告诉您我想帮助您。

I am telling you ___in ___writing that I would like to help you.

合同必须以书面形式达成协议才具有法律约束力。

The contract must be agreed upon ___in ___writing to be legally binding.