Expectation and Should

In a previous section, you learned how should can be used to express advisability:

--The front desk clerk should give us a discount.
Another way should can be used is in expressions of expectation:
--They have been working hard. They should do well.
(In this example, should means will probably.)

The past form (should have) means that the speaker expected something that did not happen:
--I haven't heard anything from them. They should have called by now.

Using Could

Could is used in two ways in English:

1.) Past ability:
--I can speak English now. I couldn't when I was a child.
2.) Polite questions:
--Could I borrow your car?
--Could you speak slower?
--Could we check out later?

Note that could is the simple past form of the verb can. The negative form of it is couldn't (could not).

Expressing Necessity:

Must/Have To/Have Got To
Must and have to both express necessity:
--You must fill out this form.
--You have to pick up Mr. Roberts.
In some situations, must is more urgent or stronger than have to:
--You must be here for your appointment on time. I have a busy schedule today.
--You have to take another course next year.
The expression have got to is similar in meaning to must and have to but is reserved for spoken English:
--I have got to study more. (i.e., I must study more.)

Using May and Might

Two other important modal verbs are may and might. The two are used interchangeably in standard American English to express probability or possibility:
--You may be wrong.
--You might be wrong.
--The attorney might be late.
--The attorney may be late.
The past form is expressed as follows:
modal (may/might) + have + past participle
--I may/might have left my glasses in the restaurant.
NOTE: Must can also be used to express probability:
--The plane must be leaving now.