Contrastive Conjunction and Modal Perfect

Modals in the past form are used in the past time.
They are could, would, should, and might. Formula of modals is:

Modals + verb 1 + O/C.
Can be used in every tenses.
a.    Could is the past tense of can
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.

b.    Would is the past tense of will
     The modal auxiliary would is  used to express a reapeated action in the past. When would is used in the past, it means “used to”, for example:
When i was younger, I would run two hours every day.
This means that I used to run two hours every day and now i don`t.  to express more polite  (request).
 Would you (please) feed the dog?
When would is used in the present, it show more polite. For unreal condition
-Mary would go to the Caribbean, but she doesn`t have enough money.(present)
- Mary would have gone to the Carribbean, but she didn`t have money.(past tense). Would, can is used in the past and present.

c.    Might is the past tense of may.
-Is used  to express of granting or seeking permission.
    Might, I leave class early?
-To express future possibility.
    She might be my advisor next semester.
-To express past possibility.
   Desty came late this morning, she might have missed the bus.
-  Might is considerably more tentative than may

d.    Should is the past tense of shall.
-Most commonly used to make recommendations or give advice. It can also be used to express obligation as well as ecpectation.
Example:
-When you go to Berlin, you should visit the palaces in Postdam. (recommendation)
-You should focus more on your family and less