These words are similar, but have subtle differences and a couple different usages that are important to understand. Both of them are used alone as a helper verb, and are followed directly with a verb without the word “to”.
Obligation versus Recommendation
Here are two example sentences that highlight the difference:
- You must pay for a ticket before you enter.
- You should buy a ticket before you go.
In these cases, the “must” is much stronger than the “should”. “Must” indicates an obligation or a necessity, without any other option. In this example, you cannot enter until you pay for the ticket. It is not a suggestion, it is mandatory.
The “should” is softer, and is a suggestion. Perhaps it is possible to buy the ticket when you arrive, but maybe there is a long line and it is better to buy it in advance. “Should” means “It is a good idea and recommended to buy a ticket before you go”.
For giving suggestions, you can use these examples:
- You ought to check the weather forecast before you go to the game.
- You should check your flight status before you go to the airport.
- You (had) better bring an umbrella in case it rains.
For indicating a necessity or obligation, you can follow these patterns:
- You must show your covid vaccination card before attending the event.
- You have to buy a ticket before entering the stadium.
- You need to create an account before you can access the information.
Recommending versus empathizing
When we imagine how another person feels, we use “must”.
- You must be tired after all that work.
- Your feet must be sore after running the marathon.
- You must be very proud of your son.
If we tell a person they “should” do or feel something, then it indicates they are not already doing this and should make a change.
- You shouldn’t eat as much salt with your high blood pressure.
- You should be happy it is raining, because it has been too dry lately. (Perhaps the person was complaining about the rain. Here we use “should” because we are suggesting they change their attitude.)
- You must be happy to see the rain, since it has been so dry lately. (Perhaps we are talking to a farmer who has been waiting for rain. In this case, we are guessing that they already are happy.)
Past tense – Logically inferring versus unfulfilled expectation
If we refer to things that happened in the past, we can use the combinations “must have” or “should have”. These also have subtle differences.
If we aren’t sure about a situation and are making a logical guess or conclusion, we can use “must have”:
- I can’t find my keys. I must have left them at the restaurant.
- I thought Jim would be here. He must have left already.
- The ground is very wet. It must have rained earlier.
If a situation did not go as expected or we wish we had made a different decision, we use “should have”:
- I didn’t know Amy was coming. You should have told me earlier.
- They arrived at the party late. They should have left earlier.
- It is was very hot in Spain in August. We should have gone in March.
