modal verbs

Can
"Can" is one of the most commonly used modal verbs in English. It can be used to express ability or opportunity, to request or offer permission, and to show possibility or impossibility.

Examples:

I can ride a horse. ability
We can stay with my brother when we are in Paris. opportunity
She cannot stay out after 10 PM. permission
Can you hand me the stapler? request


Could
"Could" is used to express possibility or past ability as well as to make suggestions and requests. "Could" is also commonly used in conditional sentences as the conditional form of "can."

Examples:

Extreme rain could cause the river to flood the city. possibility
Nancy could ski like a pro by the age of 11. past ability
You could see a movie or go out to dinner. suggestion

Have To"Have to" is used to express certainty, necessity, and obligation.

Examples:
This answer has to be correct. certainty
The soup has to be stirred continuously to prevent burning. necessity
They have to leave early. obligation


Must"Must" is most commonly used to express certainty. It can also be used to express necessity or strong recommendation, although native speakers prefer the more flexible form "have to." "Must not" can be used to prohibit actions, but this sounds very severe; speakers prefer to use softer modal verbs such as "should not" or "ought not" to dissuade rather than prohibit.

Examples:
This must be the right address! certainty
Students must pass an entrance examination to study at this school. necessity
You must take some medicine for that cough. strong recommendation


Might"Might" is most commonly used to express possibility. It is also often used in conditional sentences. English speakers can also use "might" to make suggestions or requests, although this is less common in American English.

Examples:
Your purse might be in the living room. possibility
If I didn't have to work, I might go with you. conditional
You might visit the botanical gardens during your visit. suggestion



Should"Should" is most commonly used to make recommendations or give advice. It can also be used to express obligation as well as expectation.

Examples:

When you go to Berlin, you should visit the palaces in Potsdam. recommendation
You should focus more on your family and less on work. advice
I really should be in the office by 7:00 AM. obligation
By now, they should already be in Dubai. expectation