I regularly wore a suit to work every day.
I would wear a suit to work every day.
They're not here - it's possible they missed the email.
They're not here - they could have missed the email
We believe it's important that you try harder - for your own good.
You must try harder.
It's important that we hand in the report on Wednesday. (We...)
We have to/'ve got to hand in the report on Wednesday
I came to the meeting yesterday but they didn't discuss my department. (I...)
I needn't have come to the meeting yesterday.
I didn't reply to your email because it wasn't necessary.
I didn't need to reply to your email.
I recommend that you don't arrive late again.
I wouldn't arrive late again if I were you.