Could vs. Should

conditional of can

Used as a past subjunctive (contrary to fact).

Example: ‘I think he could do it if he really wanted to.’; ‘I wish I could fly!’;

Used to politely ask for permission to do something.

Example: ‘Could I borrow your coat?’;

Used to politely ask for someone else to do something.

Example: ‘Could you proofread this email?’;

Used to show the possibility that something might happen.

Example: ‘We could rearrange the time if you like.’;

Used to suggest something.

Example: ‘You could try adding more salt to the soup.’;

Something that could happen, or could be the case, under different circumstances; a potentiality.

Was, should be, or would be, able, capable, or susceptible. Used as an auxiliary, in the past tense or in the conditional present.

past of can

used to indicate possibility

Example: ‘I would go if I could afford it’; ‘they could be right’;

used in making suggestions or polite requests

Example: ‘could I use the phone?’; ‘you could always ring him up’;

used to indicate annoyance because of something that has not been done

Example: ‘they could have told me!’;

used to indicate a strong inclination to do something

Example: ‘he irritates me so much that I could scream’;

(auxiliary) Be obliged to; have an obligation to; indicates that the subject of the sentence has some obligation to execute the sentence predicate or that the speaker has some strong advice but has no authority to enforce it.

Example: ‘What do I think? What should I do?’; ‘You should never drink and drive.’; ‘You should always wear a seat belt.’;

(auxiliary) ought to; speaker's opinion, or advice that an action is correct, beneficial, or desirable.

Example: ‘You should brush your teeth every day.’; ‘I should exercise more often, but I'm too lazy.’;

(auxiliary) Will be likely to (become or do something); indicates a degree of possibility or probability that the subject of the sentence is likely to execute the sentence predicate.

Example: ‘When you press this button, the pilot flame should ignite.’; ‘You should be warm enough with that coat.’;

Used as a variant of the present subjunctive.

Example: ‘If I should be late, go without me.’; ‘Should you need extra blankets, you will find them in the closet.’;

(auxiliary) shall

Example: ‘I told him that I should be busy tomorrow.’;

A variant of would when used with first person subjects.

Example: ‘I should imagine that everything is fine right now.’; ‘I should be lucky if I were you.’;

A statement of what ought to be the case as opposed to what is the case.

Used as an auxiliary verb, to express a conditional or contingent act or state, or as a supposition of an actual fact; also, to express moral obligation (see Shall); e. g.: they should have come last week; if I should go; I should think you could go.