A) Modal verbs are verbs we use to express different meanings, for example:
* Obligation
* Request
* Ability
* Possibility
* Suggestion
* Permission
* Advice
* Prohibition

B) These verbs must be used according to some rules:

1. They do not accept: -S, -ING or - ED suffixes;
2. They are usually followed by a BARE INFINITIVE (infinitive without TO), except for ought to;
3. In questions, they come BEFORE the subject;
4. They do not have TENSES, usually referring to PRESENT or FUTURE.

They may be resumed as follows:

CAN/ COULD - for permission, request, expressing ability or lack of ability, possibility or refusal.
MAY/ MIGHT - possibility (informal/formal), asking for or giving permission.
WILL/ WOULD - predictions, showing intentions/ expressing habits, describing hypothetical situations.
MUST - strong advice/ orders, making deductions (obvious conclusions).
OUGHT TO - strong moral advice or obligation.
SHALL/SHOULD - asking and giving suggestions or advice (informal/ formal).

EXERCISES:

1. Today, most teenagers [?] use computers easily.
2. Many of them [?] use the internet for research purposes.
3. However, experts say that parents [?] teach their children the dangers of using the www.
4. With projects like CRIE, teachers [?] update their strategies in order to encourage the use of new technologies.
5. [?] you please help me creating an e-mail address?
6. Students [?] talk inside the library.
7. [?] we enter the site right now, teacher?
8. Scan your files with the antivirus, they [?] be infected!