e.g. In the UK and New Zealand you must drive on the left.
This doesn't mean you do drive or you can drive or you want to drive!! It simply means it's the law! Everyone does it!
So 'You should drive on the left' is not correct! It can't be. You can't give yourself advice about it, it's the law!
Practice Exercise of modal auxiliaries: can and can't, must and mustn't, should and shouldn't, will and won't, would and wouldn't, might and mightn't, may and may not
1. You.......watch so much television because it's bad for your eyes. (In this example the sense tells you to put 'shouldn't' or 'mustn't'.)
2. The weather forecast says it........rain tomorrow. (In this example you can choose 'will', 'might' or 'may', according to whether you believe it 'will' (100%), it 'might' (70%), or it 'may' (50%) rain tomorrow!)
3. I ....... eat fast food because it makes me fat - but I do.
4. It.....be a good idea to learn a few phrases of the language before you travel to a foreign country because the people there.....not speak your language. (This is a suggestion but you can't use 'should' here because 'should be' has an other meaning - 'should be + adjective' gives advice - and it is not used here for giving advice!)
5. She.......want to be anything other than a writer because it's the only thing she's ever wanted to be. (You are imagining things to be different here).
6. It......be fun to be a famous actor and play lots of different parts. (Here you are making a deduction, or you are supposing, you don't know if it's true or not).
7. Although surgeons......often save lives, they don't always want to if the patient is very ill - but it is their duty to do their best.
8. You.......smoke anywhere in this building.
9. People .....find it hard to believe but the Queen Mother (of England's Queen) was still alive and working at 101 years old.
10. ......you be happy on a desert island?
11. I ...., if I had everything I needed: water, food, etcetera.
1. shouldn't (it's not against the law, it's just strong advice) 2. won't 3. shouldn't (as in 1 above) 4. might 5. wouldn't 6. must 7.can 8. must not 9. may 10. would (or could) but 'would' is for speculating i.e. imagining if things were different, and 'could' is asking about possibilities 11. might
See The A - Z of English Grammar & Usage for the rules.