Must and have to

You drive on the left in Ireland.

You wear formal clothes in our office, we are very relaxed about things like that.

I go now because I have a meeting in five minutes.

You do that, it's against company policy.

According to the contract, we deliver the project by Friday.

According to the regulations of the country, we trade without a license.

You go to the meeting if you don't have time.

You dress formally for the interview to make a good impression. Definitely no jeans.

I do a bit more work so I'll be leaving the office later tonight.

You call me 'Mr Smith'. You can call me 'David'.