What is and when do I use the subjunctive?

There is no common equivalent in English except for when you say "If I were to...". This shows an element of doubt in this phrase, meaning we don't use the conventional "I was". In French therefore, the subjunctive is used to show doubt, emotion or desire mostly. For example 'vouloir que' denotes desire therefore "I want you to know" would be translated as "Je veux que tu saches" instead of the normal "tu sais". I find it best to just certain phrases which use the subjunctive to try and get them into written or spoken assessment, as there are too many exceptions and complications in just following a rule.