1. Core modal verbs have only one form. They have no to-infinitive form, -ingform, past form or - d form. We have to reword what we want to say by using other expressions:
i We'd love to be able to see the The Red Fort one day.
Not: We'd love to can be able to see the The Red Fort one day.
ii She had to sell her farmhouse.
Not: She musted sell her farmhouse
2. Modal verbs are placed first in the verb phrase (after the subject) and are followed by a verb in the base form. The next verb may be a main verb or an auxiliary verb (be, have):
3. Modal verbs do not change form for tense or person. Modal verbs can be followed by the substitute verb do:
A: They thought he might sell his car.
B: Yes, he could do.
4. Modal verbs cannot be used with another modal verb:
Mountaineering can be difficult.
Not; Mountaineering can might be difficult.OR Mountaineering might can be difficult.
5. Modal verbs always go before other verbs in a verb phrase:
[in a restaurant after a meal]
I think the bill could be expensive.
Not: I think the bill could expensive.
6. Modal verbs do not need an additional auxiliary in negatives or questions.
For example: Must I come? (Do I must come? Is wrong)