Modal verbs: can vs must: How and When to Use in English

You must stop, which expresses an obligation to stop. Example: you must follow the rules.

You could say you can stop working now to give permission, but this sign shows an obligation.

Mustn't is the short negative form of must that we use to express prohibitions, but this sign shows the opposite! Example: you mustn't smoke here.

You can swim because the sign shows that swimming is permitted. Example: you can bring your own food to the party.

You must swim means that you are forced to swim, while this sign simply shows that you are allowed to do it.

You cannot swim means that it is not possible to swim in the area, but that's the opposite of what you see here!

You can't take pictures. This is a common sign to avoid camera flashes in theatres and museums. Example: you can't drink when you are 12.

Use must to oblige somebody to do something, not to inform them that it is not possible to take pictures. Example: you must bring a valid document.

You can take pictures means that it is possible to take them, but this sign shows the opposite!

You mustn't smoke, so it is prohibited to smoke in the area where this sign is. Example: you mustn't pick flowers in national parks.

You must smoke means that you are required to smoke… It's not great for your health and not what this sign shows!

You can smoke for example in certain designated areas at the airport, but this sign shows the opposite.

You can park here, so it is ok to leave your car. Example: they can visit us.

You must park here means that you are forced to do it, while this sign only expresses that it is possible to leave your car here.

If you can't park the sign has a red diagonal line through it and you shouldn't leave your car there, but in this case you can park.

You can't use your phone so do not make calls when you see this sign. Example: you can't stand when the plane is taking off.

Use can to say that it is possible to do something, but here it is not possible to use your phone. Example: she can come with you.

You must use your phone means that you are required to do so, but in this case the sign shows that it is not possible.

You mustn't drop garbage. This is a prohibition, which is useful to keep the area clean. Example: you mustn't tell lies.

Can is used to talk about actions that you are allowed to do, but here it's the opposite. Example: you can clean your room.

Must expresses something that you have to do, but here you do not have to drop garbage. Example: you must help your parents.

Dogs can enter. Example: can I help you?

Dogs must enter is an obligation. This sign only shows permission. 

Dogs cannot enter for example in some areas where there is a lot of exposed food, but if you see this sign they can!

People cannot enter, for example in some dangerous areas. Example: you cannot come by car because you cannot drive.

This is not what the sign shows! Must is used for obligations. Here, entry is not permitted. Example: you must leave by midnight.

People can enter is the opposite of what this sign represents. Example: you can come back tomorrow.