MODAL VERB CONCEPTS

About to start checking on modal verbs and their uses and concepts, from today on there are going to be a series of posts devoted to the topic: MODAL VERBS.

Remember the basics:

MAIN CHARACTERISTICS:

1-Each verb has its different concept meaning or function. (Look at the chart above)

2- These verbs aren't conjugated. 
No "-S" in the third person singular present.  He can swim. (Not: *He cans swim)
They lack some verb forms, which are covered by semimodals of a similar meaning.
  He had to leave. (Not *He musted leave. Must doesn't have a simple past form)
  He will be able to do it. (Not *He will can do it. Can doesn't have a future form)
[Semi-modal verbs that are used instead of the modal are:
HAVE TO for MUST (obligation)
BE ABLE TO for CAN (ability)
BE ALLOWED TO for CAN, MAY (permission)]

3- They work as auxiliary verbs in the NEGATIVE and QUESTIONS.
  Can I open the window? No, you can't open the window, it's cold.

4-  These verbs are always followed by INFINITIVE WITHOUT TO.
  He can swim. (Not * He can to swim)
Except OUGHT TO.  You ought to go to the doctor's.

5- PERFECT MODALS: past modal verb+HAVE+Past Participle refer to the past.
She hasn't arrived yet. She might have missed the bus.
You failed the exam. You should have studied more.

All the posts about modal verbs with the tag Modal Verbs.