As a non-native, I often used to get confused with the usage of word - would , so I decided to demystify its usage for everyone like me through following article. I spent some time in exploring usage of would and I am sharing what I understood.

Readers press kudos or advise change through comments.

Usage of would can be divide into following 4 basic categories.

1. Past would :
To describe repetitive habit or action, which has stopped in the past. would = used to - interchangeable in most of the cases.
She used to bring me gifts, when she came to my house. (now she do not)
She would bring me gifts, when she came to my house. (now she do not)
You would always take your surfboard when you went to the beach. (now you do not)
would you always take your surfboard when you went to the beach ?

Note : Would can not describe a past state.
She would be shy, but now she is outgoing. (wrong) - shy is a state (adjective) not a repetitive habit or action.
She used to be shy, but now she is outgoing. (Correct)
She was shy, but now she is outgoing. (Correct)

2. Conditional would:
In if.. then clause - to describe hypothetical subjunctive case. (In Hindi Hypothetical Subjunctive : agar aisa hota to, kya? )

Present: If I were a rich man, I would buy a Mercedes Benz.

Future: I will not shift my furniture tomorrow. If I had, then I would buy you lunch.
I have changed my plan, and now I will not shift my furniture tomorrow, and I am just presenting a unreal situation by saying "I would buy you lunch".

Refer this link to understand future hypothetical subjunctive case mentioned above. http://www.englishpage.com/conditional/ ... ional.html

Past: If you had learnt Chinese, then you would have received interview calls from China.

3. Future in the past:
"I knew"(past)-->---(would)---<--present-->--- future.
I knew you would help me.
I knew he was going to go to the party. (PLAN) -- "was/were going to" works like would but they are not interchangeable.
I knew Julia would make dinner. (VOLUNTARILY)

4. To express desire or preference:
I would like to come to your party.
Would you like to sit here ?

These are 4 basic categories from GMAT perspective one must be aware of. Refer following question to bolster your understanding.

A recent study on Lyme Disease suggests that people who are bitten by ticks, especially by the lone star tick, develop a severe allergy to meat that scientists believe to be turning into an epidemic

a.to be
b.would be
c.is
d.it is
e.it would be

Test choice B.
Try to categorize the requirement of would under 4 categories, which I presented above.
Category 2. if..then.. conditional (No) cause and effect not expressed.
Category 3. Future in the past (No) Past tense is not used at all in this sentence.
Category 1. Past repetitive action (No) because we are not talking about any repetitive habit or action of past, which stopped in the past.
Category 4. Desire (No)

Definitely choice B is not fitting into this sentence. Rest of the choices except C are incorrect for other reasons.

I hope this article helps clear some of your doubts regarding usage of would. Please suggest if any correction is required or share SC questions on this thread related to usage of would, so that we all can refine our understanding further.
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Thanks GmatDestroyer2013,

Sure, even I want test these concepts extensively on official questions.
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First of all, really helpful Usage of "Would"

one doubt, is this a correct example. Future: If you helped me tomorrow, then I would buy you lunch.

shouldn't it be If you help me tomorrow, I will buy you lunch.

bit confused if the use of (helped + tomorrow) is correct in above example ?
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Hi Piyush,
Need your help on the below-
To develop more accurate population forecasts, demographers would have to know a great deal more than they do now about the social and economic determinants of fertility.
I have a query with the use of would. Would is used correctly when there is a reference of another past event.
Ex- If I had 10 million $, I would buy the luxury car.
Ex- The scientist believes that the machine would be wonderful.

The above example is wrong as 'believes' is in present. If believes is replaced by 'believed' the ex becomes correct.
So, how can would go hand in hand with do?