- Must
We use must to express something that you or other people think it is a serious obligation. This form is used only in the present and future.
Examples:
We must pay our taxes.
Children must obey their parents.
We use must to express speculations, when you are 100% (or almost 100%) sure that something is correct or true.
Examples:
It must be raining. Listen!
My father must be at the office. His car is in front of his office.
These modals follow the same patterns:
Affirmative pattern
Subject + modal auxiliary + verb in simple form + complement.
I can speak English well.
We could run fast 10 years ago.
We must pay our taxes.
We should study everyday.
Negative pattern
Subject + modal in negative + verb in simple form + complement.
I can´t play the guitar.
They couldn´t swim 5 years ago.
We mustn´t destroy trees.
She shouldn´t drive so fast.
Interrogative form
Modal auxiliary + subject + verb in simple form + complement + ?
Can you dance salsa?
Could he drive 5 years ago?
Must we enter now?
Should people save water?