As Mikawa Ossan said, the 'ought to' and 'should' constructions mean that the research was bad but the option of not re-doing it exists. If the speaker uses 'I have to', this means the work was done so poorly that the speaker cannot forgive the mistake, the work must be done again and there is no choice.
As for this:
3)You ( ) work harder if you would want to pass the exam.
The structure 'would want' is used when discussing the future hypothetical in most cases.
i.e.
a) I'd want one too if they came in blue.
(but they don't, so I don't want one)
b) I'd want to see the car before making a deposit.
(but I'm currently not interested in buying a car so I do not want to see one right now)
c) She'd definitely want to go for a ride once you'd bought it.
(but you haven't bought the car yet so she doesn't want to go for a ride yet)
d) You should really go, he'd want you to be there.
(if he was still alive, but he's dead so technically he can't want anything, we are only speculating)
In all these cases the wanting is not 100%, a condition exists whether spoken or unspoken. Grammatically, the 'wanting' must be clearly modified. The example you used can't use 'would' if the only other information is 'you should study hard', because this is not a condition that affects this persons desire to pass the exam. It must be something like:
I would want to pass the exam if it meant I'd be accepted at Senshu University.
or
If it meant that I'd be accepted at Senshu University then I'd want to pass the exam.
In your example 'You should work harder if...', the context suggests that we already know he wants to pass, there is no question about whether he
would want to pass, the question is what does he have to do in order to succeed?
To answer the original question, the choices are the same as per your original example. 'Have to study harder' means this is the most important thing, it's something you must do. 'Should study harder' is like a suggestion, studying harder is recommended but perhaps there are other options. 'Ought to study harder' is similar to 'should study harder', its a suggestion.