He’s too chicken-hearted to defend his beliefs in public.

他膽子太小，不敢在公眾面前為自己的信仰辯護。

Cheat on somebody:

給某人戴綠帽

例句：

She cheats on her husband.

她給她老公戴綠帽。

Have an affair with somebody:

跟某人有曖昧關系

例句：

You have an affair with him.

你和他有曖昧關系。

Two-time:

對(情人或者配偶)不忠

例句：

Are you sure he’s not two-timing you?

你肯定他沒有背著你另有所愛?