Mr.Maru: Hey Sparky! Are you ready for dinner?
Sparky: Yeah, what are we having?
Mr.Maru: Bacon cheeseburgers.
Sparky: Oh, bacon? I love bacon.
Mr.Maru: I can't find the bacon though.
Sparky: Really? That's interesting.
Mr.Maru: Yes, very interesting. It was here when I left. Did you eat it?
Sparky: Me?! No. You must protect the bacon better.
Mr.Maru: Why do you smell like bacon then?
Sparky: There should be a mistake. The bacon thief should have rubbed some grease on me.
Mr.Maru: Did you see the bacon thief?
Sparky: I was asleep. He should have stolen the bacon then rubbed the grease on me while I was sleeping. Dirty bacon thief!

What is the difference between must and should?  When do we use must and when do we use should? Let's talk about some examples and the basic rules.

Basic Use of must and should

The words 'must' and 'should' are called modal auxilary verbs or 'modals' in English.  They help other verbs.  The modal 'must' is used to talk about when something is certain.  There is a lot of evidence, clues or information to support the statement.  It can also be used when there is no other explaination for something.  For example,

He is smiling and laughing.  The joke must be really funny.

That necklace has lots of big diamonds on it. It must be expensive!
I looked everywhere in my house for my umbrella, but couldn't find it.  I must have lost it.

The modal 'should' is used to talk about an obligation, duty or responsibility.  For example,

I should wash my car.  (Keeping my car clean is my duty.  Society wants me to clean it because it looks nicer.)
The bathroom is hard to find.  They should have more signs.  (The owner of this building has a duty.  That duty is to help people find the facilities (bathroom).)

The modal 'should' is also used for probablity.  This is usually used when we have some evidence or proof.  For example,

She should win the gold medal.  She is a fast runner. (Because she is a fast runner is more likely to win or has a better chance of winning the gold medal.)
The manager went to a meeting.  He should be back in an hour. (I am pretty sure the manager will be back in an hour because he usually has a meeting for one hour.)

The difference between 'must' and 'should'

The main difference between these two modals is that 'must' is more probable or more likely to happen than 'should'.  For example,

You must clean the floors. (I told you to clean the floors.  If you don't clean the floor, I will be angry.  I will force you to clean the floor.)
You should clean the floors. (I will feel better if you clean the floors, but you don't have to.)

There must be something wrong with this car. (The car is not running as usual.  The best explaination is that there is something wrong with it.)
There should be something wrong with this car. (There is some evidence for me to think there is something wrong, but I can't prove it a 100%. For example, the dealer is selling the car for a cheaper price than usual or the car hasn't been started for a long time.  I'm less certain because I did not drive the car or experience anything wrong.  I'm only guessing after seeing the evidence.)

Do you understand?  Let's take a short quiz to check.

Must vs. Should Quiz

1) You ________ take off your shoes when you enter a Japanese house.  It is a strong rule that everyone follows.

A) must
B) should

2) It _____ be raining out.  I hear the rain drops on the roof.

A) must
B) should

3) You _________ visit Paris.  I think you would have fun.

A) must
B) should

Answers

1) A) - There is no other choice.  It is a strong rule, so you must follow it.

2) A) - There is no other explaination.  If you hear rain drops it is raining.

3) B) - You have a duty to yourself because you will have fun.

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