Ring a/any bell: (informal) nghe quen thuộc với bạn.

The song rings a bell but I can't remember its name.

Be under weather: (informal) cảm thấy hơi mệt trong người / không khỏe như bình thường.

Can I take a day off? I'm feeling under the weather today.

Rule of thumb: cách làm một việc dựa trên kinh nghiệm từ trước chứ không phải do đo đạc cụ thể.

As a rule of thumb, you should give up your seat to disabled or elderly people.

A good rule of thumb is that a portion of rice is two handfuls. 

To find my feet: thích nghi với môi trường mới và có thể hành động một cách tự tin và độc lập.

My family have just moved to Germany so I'm still finding my feet with everything.

A hot potato: (informal) một vấn đề khó khăn và gây nhiều tranh cãi.

Education is always a social hot potato in Vietnam.

At the drop of a hat: ngay lập tức, ngay tức khắc.

If you need anything, just call me. I'll be there at the drop of a hat.

The best of both worlds: có được lợi thế / ưu thế của hai hay nhiều lựa chọn cùng một lần.

This full-day tour will take you to a small village at the foot of a mountain. You can enjoy the best of both worlds.

She works in the city and lives in the country, so she gets the best of both worlds.  

Be a far cry from sth: rất khác với.

Working as a journalist is a far cry from studying at university about journalism.

This flat is a far cry from the house they had before.  

Give someone the benefit of the doubt: ấp nhận là những điều ai đó nói hoặc làm là đúng vì bạn không thể chứng minh là họ sai.

There was something fishy going on there, but I had no choice but to give her the benefit of the doubt.

She said she was late because her flight was canceled, and we gave her benefit the of the doubt.  

Hit the nail on the head: đánh đúng / nói đúng trọng tâm(describe exactly what is causing a situation or problem)

She hit the nail on the head when she said that our company needs a stronger leader.

I think Nick hit the nail on head the when he said that what's lacking in this company  is a feeling of confidence.  

In the heat of the moment: thực hiện một hành động hoặc nói gì đó lúc thiếu suy nghĩ do đang cao hứng hoặc tức giận.

In the heat of the moment, we said terrible things to each other.

Hold/keep s.o/s.th at bay: ngăn cản ai đó đến gần hay ngăn một vấn đề co ảnh hưởng tiêu cực.

I hate reptiles! Snakes. crocodiles. Keep them at bay!

The whole nine yards(informal): tất cả mọi thứ trong 1 tình huống nào đó.

Mary always goes the whole nine yards whenever she cleans her house.

When I was little, my family always had a lot of pets- dogs, cats , hamsters, fish, rabbit- the whole nine yards.  

The weather was terrible but I wanted to go the whole nine yards and get to the top of the mountain.

Sweat blood(informal): làm đổ mồ hôi sôi nước mắt(to make a great effort)

We sweated blood to get the work finished on time.

I've been sweating blood over this report.

Over the moon: cực kỳ hạnh phúc và hào hứng.(to be very pleased)

They are over the moon about the 50% discount for students.

She was over the moon about/with her new bike.

A drop in the ocean: muối bỏ biển / thứ gì đó quá nhỏ để tạo nên ảnh hưởng.(a very small amount compared to the amount needed)

Ten participants are just a drop in the ocean compared to the number of people we need for the event.

My letter of protest was just a drop in the ocean.

Closed book: một chủ đề hay người nào đó mà bạn hoàn toàn không biết gì.(a subject about which you know or understand nothing)

I give up - Parapsychology will always be a closed book to me.

I'm afraid physics will always be a closed book to me.

By the book: làm theo đúng hướng dẫn và quy luật.(exactly according to rules or the law)

The teacher asked us to do the test by the book.

The to make we've done everything by the .  

Bread and butter: cơm áo gạo tiền (a job or activity that provides you with the money you need to live)

Gardening is my bread and butter right now.