QUICK TIP: Would, could OR should?

What's the difference between would, could and should?  A reader of mine sent in this question, so let's check it out.  Because there are so many aspects to this question, this post will only focus on the past and present tenses of would, could and should.

PAST and PRESENT Tenses

1.  WOULD--WILL (expressing want or one's will to do something)

• Would is the past tense of will.  Often times, would is used in reported speech in the past.
• Ex. "I will go to the swimming pool after work tomorrow."
• Ex. John said that he would go the the swimming pool yesterday.
• Would is also used to express an action that happened regularly in the past.
• Ex. When I was a little girl, I would visit my grandparents in the summer in New York.

2.  COULD--CAN (expressing ability and feasibility)

• Could is the past tense of can.  
• Ex. Today I can surf big waves, but one year ago I couldn't even surf small waves. 
• Ex. He said that he could go to the party, but now he says he can't.

3.  SHOULD HAVE--SHOULD (expressing advice strongly) *Note: There is no past tense of should...there is only should have.

• Should have expresses regret, while should expresses advice.
• Ex. I should have taken an earlier flight because I was late for my business meeting.
• Ex.  You should quit smoking.

Look out for the following post on would have, could have and should have. 

Overview: Conditionals 1, 2, 3

In English, there are many ways to express situations according to a certain condition in the present and the past, as well as a varying degree of possibility.  Basically, we have the following framework to express different situations:

Conditional 1:

a) possible event that depends on a condition

b) refers to the future

c) grammar tenses=present, future

If I get into a car accident, my auto insurance will cover the hospital bills.
 (present)                                                                   (future)

He will call you if his plane lands early.
      (future)                         (present)

If the citizens elect a new president, then I will find a new job.
                    (present)                                    (future)

I will inform you if I get the job.
     (future)             (present)

Conditional 2:
a) possible OR impossible situation
b) the possible situations refer to the future, the impossible is hypothetical (use your imagination!)
c) grammar tenses=past, "would" (a modal verb)

d) *Even though we use the past tense, it DOES NOT refer to the actual past

If I won a million dollars, I would donate to SOS charities. (possible in the future)
     (past)                                 (modal verb)

If I were you, I would see a doctor as soon as possible. (impossible, hypothetical)
      (past)          (modal verb)

I would renovate the house if I had more money. (possible in the future)
   (modal verb)                          (past)

I would be King John III if I could be anybody for one day.  (impossible, hypothetical)
 (modal verb)                         (past)

Conditional 3:

a) impossible situations

b) impossible situations that refer to a finished, unreachable past

c) grammar: past perfect (had+past participle), modal perfect verb (would have+past participle)

If my mother had spoken German with me as a little girl, I would have been a fluent German speaker!        (past perfect)                                                      (modal perfect verb)

She would not have failed the exam if she had studied more. 
        (modal perfect verb)                            (past perfect)

If I had gone to the doctor a month ago, I would have recovered by now.
 (past perfect)                                                 ( modal perfect verb)

They would not have broken up if he had treated her better.
            (modal verb)                              (past perfect)