The subjunctive is used to emphasize urgency or importance or after certain expressions. We use the simple form of the verb (infintive without to) as the subjunctive form. 'I suggest that he study' (not studies) Because we also use the simple form in the present simple (for I, You, We, They) use of the subjunctive is not always noticeable. 'It is important that you try to study often ('try' here is subjunctive). Subjunctive is used after the following verbs. To advise, to ask , to command, to demand, to desire, to insist, to propose, to recommend, to request, to suggest, to urge, (that) 'The teacher insists that her students be on time.' Negative subjunctive: 'The boss insisted that Sam not be at the meeting.' Passive subjunctive: 'We suggested that you be admitted to the club.' Continuous subjunctive.' It is essential that a car be waiting for us when we arrive.' The Subjunctive is also used after expressions such as. It is best that, It is important that etc. 'Shoud' is often used instead of the subjunctive. 'The doctor recommended that she see a specialist.' 'The doctor recommended that she should see a specialist.'

Your question is way to general. http://en.wikipedia.org/wiki/Subjunctive_mood