past perfect + would have + ed-participle / past simple + would + infinitive

Could you clarify difference between sentences:

1. If I had asked people what they wanted, they would have said faster horses
2. If I asked people what they wanted, they would say faster horses

In a similar vein:

1. If you had asked me I would have followed
2. If you asked me I would follow

They're all hypothetical, aren't they?

Yes, they are all hypothetical, in the sense that they describe actions that haven't actually happened. However, the difference between the two is based on whether the event mentioned in the condition actually happened. Let me illustrate with a few examples.

The first form ("If X had asked") can be applied to situations where some entirely different action was taken, i.e. the event mentioned in the condition never happened.

• If you had asked me I would have followed. [but you didn't ask me, so you had to go alone]
• If I had asked people what they wanted, they would have said faster horses. [but I didn't ask them, so I bought fancy but useless equestrian equipment instead]
• If I had studied, I would have passed. [but I didn't study, so I failed]

Since they describe conditions where the past could have been changed, these sentence forms can be used to express regret or wistfulness as well.

The second form ("If X asked") does not presuppose any action having taken place at all. It deals purely in the hypothetical, and suggests that something is likely to happen if the condition is met.

• If you asked me I would follow.
• If I asked people what they wanted, they would say faster horses.
• If I studied, I would pass.

In all these forms, the speaker is reasonably confident that if an action was carried out, the consequences would be as he or she described.

In summary, use the first form when you want to describe a situation where a different series of events transpired. Use the second form to describe a more hypothetical scenario.

Simple: the first refers to the past, the second refers to the present.

If I had asked people what they wanted, they would have said faster horses.

This implies that I didn't ask people what they wanted and I've lost my chance to.

If I asked people what they wanted, they would say faster horses.

This implies that I haven't asked them yet, but if I were to do it right now, they'd say faster horses. I can make an experiment(i.e. actually ask them) and see if my hypothesis is true. In case of the first sentence, there's no chance to verify my hypothetical claim any more. It's all in the past.

I agree with what Mganjoo has pointed out so clearly in the post above. These sentences are normally called "if clauses", so I do not understand what difference is meant by User 20119 when he mentions "hypothetical" sentences as being different.

I would also add a different construction, in which you indicate the outcome of a past action, that is to say something which would not be true now if something else had not previously happened. An example of this is

If my grandfather hadn't emigrated to the United States, I wouldn't be living in Seattle

(if he had stayed in his native country, I would have been born there and would be living there too). In this case you have a conditional form in the main clause and a Past Perfect tense in the hypothesis, which may be introduced by if or may present subject-verb inversion with no difference in meaning.