1.It was one of mother's maxims-never look a gift horse in the mouth.

母亲的箴言之一是对于收到的礼物绝不要挑毛病。

don't look a gift horse in the mouth:

Do not unappreciatively question a gift or handout too closely.

2.Would you stop being a backseat driver? I don't want you to teach me how to drive.

你可不可以不要再指指点点了?我不需要你教我怎么开车。

He is a backseat driver, and he always tells me what to do.

他是个多管闲事的人，总是告诉我该做什么。

back-seat driver:

A person sitting (typically) in the back seat of a car issuing (unsolicited and usually unwanted) instructions to the driver.

(figuratively) An opinionated person who offers (unsolicited and often unwanted) advice on the best way to handle a situation.

3.He is a rubbernecker. You can see him at every quarrel scene.

他特别喜欢看热闹。哪里有吵架，哪里就有他的身影。

rubbernecker:

Someone who watches, a spectator, in the sense that they crane their neck as though it were made of rubber. Especially those who watch accidents.