You will use "would" in future. When it is a possibility or a wish. Corresponds to "futuro do pretérito" in Brazil.
Ex: I would like to travel to Brazil.
You will use "should" when you give an advice.
Ex:You should to study more!
"Could" is a junction of can+would. This way, we have: poderia, conseguiria.
Ex: They could date if they weren't cousins.
- in formal request like: WOULD YOU LIKE SOME CAKE?
- In conditional sentences in which its result is unlikely as: If Mary studied she WOULD pass the exam.
- In conditinal sentences in which the result is impossible as: If Mary had studied she WOULD have passed the exam.
We use SHOULD to give advice as: You SHOULD give some aspirins.
We use COULD to talk about abilities, possibilities in the past or formal request as:
I COULD swim when I was younger
I COULD go out las night but I prefer stay at home
COULD you help please?
OBS: WOULD, SHOULD and COULD are modal verbs. They change or complete the meaning of the main verb in the sentence.