Keep a tight rein on (someone or something)
To strictly watch and control someone or something.Our principal keeps a tight rein on what is being taught in the school.
Keep a straight face
To stop oneself from smiling or laughing.It was difficult to keep a straight face when the man fell off his chair.
Keep a stiff upper lip
To be brave, to face trouble bravely.The storm victims tried hard to keep a stiff upper lip in the difficult situation.
Keep a secret
To not tell a secret to others.I am trying to keep a secret about my friend's boyfriend.
Keep a close watch over (someone or something)
To guard or care for someone or something.I kept a close watch over the soup while it was cooking.
Keep a close watch on (someone or something)
To monitor or observe someone or something.The woman always keeps a close watch on her child when she is at the shopping center.
Keep a close rein on (someone or something)
To strictly watch and control someone or something.The mother was keeping a close rein on her child at the shopping center.
Keep a civil tongue
To speak decently and politely.The angry customer was asked to keep a civil tongue when talking with the sales clerk.
Keen on (someone or something)
To be enthusiastic about someone or something.My girlfriend is keen on going to a movie this weekend.
Keel over
To turn upside down, to tip over.The boat keeled over in the middle of the lake but everybody was safe.