He was too diffident to express his opinion.

他太不自信，不敢发表自己的见解。

He is diffident about expressing his opinions in the public.

他在公开场合表达自己的意见感到胆怯。

He has a politely diffident manner.

他的态度是客气中带有羞怯。

There was something shamefaced or diffident about the movement, as if the intention were to conceal any idea of stopping until the very last moment.

这人的行动有些害羞者有些胆怯，好像不到最后一刻都不打算暴露任何要停下来的想法。