Use should and shouldn’t to ask for and give advice and suggestions:
“I’ve had a really bad headache for the past week.”
“That’s not good – you should go to the doctor.”
“I want to make more friends, but I don’t know how.”
“First of all, you shouldn’t spend so much time on the computer. You should go out and join a club or start playing a sport instead!”
“I had a fight with my best friend. What should I do?”
“Hmm. I think you should call her and tell her you’re sorry.”
Use could and couldn’t for ability in the past (they are the past forms of can and can’t):
Use could to talk about future possibilities:
“Do you have any ideas for our publicity campaign?”
“Yes, I’ve got a few ideas. I could put advertisements on Facebook and Google. We could also give out pamphlets in our neighborhood. Maybe John could even contact local TV stations.”
Use could to make polite requests:
Use would to talk about unreal or unlikely situations:
In this case, would is often shortened to ‘d
Use Would you like…? to make polite offers:
“Would you like anything to drink?”
“A soda would be great. Thanks!” “Would you like to join us for dinner?”
“I’d love to, but I actually have other plans tonight.”
“Would you like to see some pictures from my vacation?” “Sure!”
Don’t use “to” after should, could, or would:
You shouldn’t to smoke.
You shouldn’t smoke.
We could to order pizza tonight.
We could order pizza tonight.
I would to buy a new car if I had the money.
I would buy a new car if I had the money.