Modal Auxiliaries

Modal Auxiliaries
Other helping verbs, called modal auxiliaries or modals, such as can, could, may, might, must, ought to, shall, should, will, and would, do not change form for different subjects. For instance, try substituting any of these modal auxiliaries for can with any of the subjects listed below.
I
you (singular)
he
we
you (plural)
they can write well.

There is also a separate section on the Modal Auxiliaries, which divides these verbs into their various meanings of necessity, advice, ability, expectation, permission, possibility, etc., and provides sample sentences in various tenses. See the section on Conditional Verb Forms for help with the modal auxiliary would. The shades of meaning among modal auxiliaries are multifarious and complex. Most English-as-a-Second-Language textbooks will contain at least one chapter on their usage. For more advanced students, A University Grammar of English, by Randolph Quirk and Sidney Greenbaum, contains an excellent, extensive analysis of modal auxiliaries.
The analysis of Modal Auxiliaries is based on a similar analysis in The Scott, Foresman Handbook for Writers by Maxine Hairston and John J. Ruszkiewicz. 4th ed. HarperCollins: New York. 1996. The description of helping verbs on this page is based on The Little, Brown Handbook by H. Ramsay Fowler and Jane E. Aaron, & Kay Limburg. 6th ed. HarperCollins: New York. 1995. By permission of Addison-Wesley Educational Publishers Inc. Examples in all cases are our own.
Uses of Can and Could
The modal auxiliary can is used
• to express ability (in the sense of being able to do something or knowing how to do something):
He can speak Spanish but he can't write it very well.
• to expression permission (in the sense of being allowed or permitted to do something):
Can I talk to my friends in the library waiting room? (Note that can is less formal than may. Also, some writers will object to the use of can in this context.)
• to express theoretical possibility:
American automobile makers can make better cars if they think there's a profit in it.
The modal auxiliary could is used
• to express an ability in the past:
I could always beat you at tennis when we were kids.
• to express past or future permission:
Could I bury my cat in your back yard?
• to express present possibility:
We could always spend the afternoon just sitting around talking.
• to express possibility or ability in contingent circumstances:
If he studied harder, he could pass this course.
In expressing ability, can and could frequently also imply willingness: Can you help me with my homework?

Can versus May
Whether the auxiliary verb can can be used to express permission or not — "Can I leave the room now?" ["I don't know if you can, but you may."] — depends on the level of formality of your text or situation. As Theodore Bernstein puts it in The Careful Writer, "a writer who is attentive to the proprieties will preserve the traditional distinction: can for ability or power to do something, may for permission to do it.
The question is at what level can you safely ignore the "proprieties." Merriam-Webster's Dictionary, tenth edition, says the battle is over and can can be used in virtually any situation to express or ask for permission. Most authorities, however, recommend a stricter adherence to the distinction, at least in formal situations.
Authority: The Careful Writer by Theodore Bernstein. The Free Press: New York. 1998. p. 87.

Uses of May and Might
Two of the more troublesome modal auxiliaries are may and might. When used in the context of granting or seeking permission, might is the past tense of may. Might is considerably more tentative than may.
• May I leave class early?
• If I've finished all my work and I'm really quiet, might I leave early?
In the context of expressing possibility, may and might are interchangeable present and future forms and might + have + past participle is the past form:
• She might be my advisor next semester.
• She may be my advisor next semester.
• She might have advised me not to take biology.
Avoid confusing the sense of possibility in may with the implication of might, that a hypothetical situation has not in fact occurred. For instance, let's say there's been a helicopter crash at the airport. In his initial report, before all the facts are gathered, a newscaster could say that the pilot "may have been injured." After we discover that the pilot is in fact all right, the newscaster can now say that the pilot "might have been injured" because it is a hypothetical situation that has not occurred. Another example: a body had been identified after much work by a detective. It was reported that "without this painstaking work, the body may have remained unidentified." Since the body was, in fact, identified, might is clearly called for.

Uses of Will and Would
In certain contexts, will and would are virtually interchangeable, but there are differences. Notice that the contracted form 'll is very frequently used for will.
Will can be used to express willingness:
• I'll wash the dishes if you dry.
• We're going to the movies. Will you join us?
It can also express intention (especially in the first person):
• I'll do my exercises later on.
and prediction:
• specific: The meeting will be over soon.
• timeless: Humidity will ruin my hairdo.
• habitual: The river will overflow its banks every spring.
Would can also be used to express willingness:
• Would you please take off your hat?
It can also express insistence (rather rare, and with a strong stress on the word "would"):
• Now you've ruined everything. You would act that way.
and characteristic activity:
• customary: After work, he would walk to his home in West Hartford.
• typical (casual): She would cause the whole family to be late, every time.
In a main clause, would can express a hypothetical meaning:
• My cocker spaniel would weigh a ton if I let her eat what she wants.
Finally, would can express a sense of probability:
• I hear a whistle. That would be the five o'clock train.

Uses of Used to
The auxiliary verb construction used to is used to express an action that took place in the past, perhaps customarily, but now that action no longer customarily takes place:
• We used to take long vacation trips with the whole family.
The spelling of this verb is a problem for some people because the "-ed" ending quite naturally disappears in speaking: "We yoostoo take long trips." But it ought not to disappear in writing. There are exceptions, though. When the auxiliary is combined with another auxiliary, did, the past tense is carried by the new auxiliary and the "-ed" ending is dropped. This will often happen in the interrogative:
• Didn't you use to go jogging every morning before breakfast?
• It didn't use to be that way.
Used to can also be used to convey the sense of being accustomed to or familiar with something:
• The tire factory down the road really stinks, but we're used to it by now.
• I like these old sneakers; I'm used to them.
Used to is best reserved for colloquial usage; it has no place in formal or academic text.

شكرا يالدكتور مؤيد والله يجزاك الجنة

thanks alot man