"Neha wants to study."
Translation:नेहा पढ़ना चाहती है।
When used with a verb (like पढ़ना in this sentence), all forms of चाहना except चाहिए mean 'want' while चाहिए means 'should' or 'need to'.
नेहा पढ़ना चाहती है। - Neha wants to study
नेहा को पढ़ना चाहिए। - Neha should study/Neha needs to study.
When used with a noun, all other forms of चाहना except चाहिए still mean 'want'. चाहिए, in this case, conveys a stronger form of 'want' (verging on 'need') or is used to highlight some form of immediacy.
राज पैसा चाहता है - 'Raj wants money' (meaning, he wants money in the undefined future)
राज को पैसा चाहिए - 'Raj needs money' or 'Raj wants money' (right now).
चाहना is a transitive verb which means it can take direct objects.
Eg: राज बहुत सारा पैसा चाहता है - 'Raj wants a lot of money'. Here, बहुत सारा पैसा is the direct object of चाहना which is why it is not in the oblique case.
Similarly, you can think of the verb infinitive पढ़ना as being a direct object of चाहना in the parent sentence which is why the oblique case form of the infinitive पढ़ने is not used.