As we know “if” its a conditional, an example its: If you never tried you will never know. Si no lo intentas nunca lo sabras. And would its kind of a past of the verb “will” wich modificated the verb. Would you come? Vendrías? Thesse two can be used together: I would visit her, if i had time Yo la visitaría , sí tuviera tiempo
Could its used to express possibility or past ability as well as to make suggestions and requests. Could you do it? Podrias hacerlo? You could fly Tu podrias volar using could with “if”: If I could fly ill make it si podria volar lo haria
Should is most commonly used to make recommendations or give advice. It can also be used to express obligation as well as expectation: You should do it Tu deberías hacerlo Used with the conditional “if” it make an advice or a recommendation with a condition: You should do it, if were good on it Tu deberías hacerlo, si fueras bueno en eso
Must express an obligation or a prohibition, something you have or not do. Example: You must go to the concert Tu debes ir al concierto Using this modal with the conditional “if” makes a conditional sentence with an obligation or a prohibition. Example: If you must go, then go si debes, pues ve If you must not go, then why did you go? Si no deberias ir, entonces porque vas?