Have to — You’re required to do something by rule, or law, or some authority, someone’s orders
For ex: You have to pay tax. You have to stand in the queue.
Negative: Don’t have to — you’re not required to do anything and you are free to do.
For ex: I don’t have to pay more taxes than required.
Need to — You’re required to do something because there’s a need, whether you like it or not. Whether someone says it or not.
For ex: I need to eat fruits for proper nourishment. You need to bathe everyday.
Negative: Don’t have to, needn’t — you’re not required to do anything and you are free to do.
Must — You think it’s important for you or someone else to do something. Not because someone tells you, not according to any law or rule or practice. You decide it’s important to do something. You feel strongly about it. It’s more your opinion.
For ex: I must exercise to stay healthy. You must meditate.
Negative: Mustn’t — You think it’s important for you or someone else NOT to do something.
For ex: I mustn’t have sweets.
Should — To give advice. You think it’d be good if you or someone else does something. Whether it’s expected by others or not. You just think it’s just the right thing at the moment. It’s still your opinion but you don’t feel strongly about it.
For ex: I should learn to drive.
Negative: Shouldn’t — You think it’d be good if you or someone else doesn’t do something.
For ex: I shouldn’t advise people who don’t ask for it.
Used to — To talk about something which we did often in the past but we no longer do.
For ex: I used to run earlier but I walk mostly these days. She used to be thinner. She’s put on some weight now.
