Would, should and could are auxiliary verbs that can be defined as past tenses of will, shall, and can. However, they are also used in other situations.
Would is the past tense of will, but it is an auxiliary verb that has many uses; it is sometimes even used is the present tense. It can be used:
- to ask questions
What would you do if you had lot of money?
Who would like to walk in the rain?
Would you like to watch a movie with me?
- to make a polite request
Would you like some coffee and cake?
Would you like to do something else?
- to express wishes or desires
I wish you were here.
I wish she would go home.
- to talk about hypothetical situations
If I won the lottery, I would buy myself a Porche.
I would love to buy a yacht and go on a trip around the world.
- to express intentions and plans
He said he would come to our party.
She said she would clean the floor.
- to express a different reaction if the past had been different
I would have gone to university if I have had the money.
I would not have missed the flight if the bus had arrived on time.
- to talk about habitual and repetitive past actions
She would clean the house thoroughly whenever her mother-in-law announced her visit.
He would always drive very slowly around that dangerous spot.
- to talk about a choice
I would stay home if I could.
We would go camping if we could, but all the campsites are closed now.
Should is the past tense of shall. However, it is an auxiliary verb which has several uses, and it is not always used in the past tense. It is used
- to express probability
I should be home by 5:00 PM.
He should be studying for his exams.
If we walk fast, we should be home by 5PM.
If I study hard this week, I should pass the exam.
- to ask questions
Should I go on the school trip on Friday?
Shouldn’t you be studying for your final exam?
- to show obligation or give recommendation
You should give up smoking.
You should do more exercise.
We should go for a swim tomorrow.
- to talk about possible future event or hypothetical situation
If I should find your keys, I will call you.
Should you wish to drive, you may take the car keys.
Could is the past tense of can. However, it is an auxiliary verb that has several uses, and it is not always used in the past tense. It is used
- as the past tense of can
I could write when I was six.
Not many people could go to university in the 19th century.
- To ask questions and make polite requests
Could you open the window?
Could we go now? I am tired.
- To show possibility:
I could work extra hours to save up some money.
Whose textbook is it? It could be his.