This is a great and important question! I apologize for this long explanation. If there is anything you do not understand, just ask and I will try to explain in simpler terms. Can is used in present tense, but shouldn’t be used in a second conditional statement. Ex: If I had $100, I can buy 20 tacos. This sentence doesn’t sound right, and isn’t grammatically correct. First, it’s important to know what tense the verbs should be in for a second conditional statement. Second conditional statements are made up of 2 parts: 1) A dependent clause: “If I had $100...” In the dependent clause, the verb is in past tense. (If I HAD $100..., If I ATE something spicy..., If I RAN 50 miles...) 2) An independent clause: “...I could buy 20 tacos.” In the independent clause, you use “could/would/should” then a verb in present tense. (I COULD BUY 20 tacos, I WOULD FEEL sick, I WOULD BE tired.) So now the difference between could/would/should: COULD is used to express what someone CAN do or is capable of doing in the future or in a hypothetical (imaginary) situation. Ex: If I had $100, I could buy 20 tacos. I am not saying that I WILL buy tacos, but that I CAN if I want to: I COULD. WOULD is used the same, only for if someone WILL do something in the future or in an imaginary situation. Ex: If I had $100, I would buy 20 tacos. I am saying that if I had the money, this is what I will do with it: I WOULD SHOULD is used for advice, or saying what you or someone else OUGHT to do. Ex: If I had $100, I should buy 20 tacos. You’re not saying you will or you can, you’re saying that it’s what you OUGHT to do, or perhaps it’s the best way to spend the money: I SHOULD In this example, I think it would be better to say: If I had $100, I should not buy 20 tacos. I hope this helps!