SHALL / WILL          SHOULD / WOULD

    
     Shall - Will

     Shall and will are both modal verbs primarily used to express the future tense.

     In informal English, the Simple Future is usually conjugated entirely with the auxiliary will,
     particularly in American English.

     In more formal English, there is a rule which states that, as shown in the table below,
     ♦  the auxiliary shall should be used in the first person (I/we),
     ♦  and the auxiliary will should be used in the second person and third person (you, he/she/it, they):  

     However, shall currently tends to be falling out of use,
     but it continues to be used with I and we for offers and suggestions:

          * Shall I open the window?
          * Shall we dance?

     Should/would

     Should is the conditional form of shall.
     Would is the conditional form of will.

     In informal English today, the conditional tense is usually conjugated entirely with would :
        - If I had enough money, I would buy a new car.

     Should is used:
        - to give advice :                    You should take regular exercise.
        - in hypothetical situations :   Should you need any help, just call me.
        - to give tentative opinions :    I should think the cost will be about £100.

      (see also: should-ought to)