Will and Would :
Will is used in future tense. For example:
I will go to school - Mai school jaunga.
Would is the second form of Will.
It is used in sentences which show past. For example:
I promised him yesterday that I would go to school - Maine use Kal wada kiya tha ki mai school jaunga. This sentence shows past. Therefore we used would instead of will. But we can use would in normal sentences too. For Example:
I would go to school - This is not wrong. But it is better to use would in sentences which show past.
Can and Could. Examples:
I can walk - Mai Chal sakta hu.
I could go to school - Mai school ja sakta tha.