When should we use the subjunctive tense in Spanish?

In English, less and less people use the subjunctive, so it can be hard for us to recognise when it is needed in Spanish. However, we can generally follow this rule: when there is any level of doubt or uncertainty, the subjunctive must be used. When certainty exists, the indicative ('normal') form is needed.

For example: 'El va a Peru mañana' = indicative ('va') because the speaker seems confident that he is going to Peru tomorrow. However, if we alter this to be 'Es dudoso que el vaya a Peru mañana', we need the subjunctive ('vaya') as we are not sure whether or not he will go to Peru tomorrow - there is a level of uncertainty. Other phrases such as 'It is good that...' or 'It is important that...' also call for subjunctive to be used.