Subjunctive

Subjunctive is rather insignificant in modern English. Most likely you might come across this form in American English, and most probably in formal texts.
Form
In most cases, subjunctive and indicative forms of a verb are the same. So often you would not notice whether a verb is used in subjunctive or indicative mood. The only indicator for subjunctive is that no ‘s’ is added in 3rd person singular and that the verb ‘be’ remains ‘be’ for all forms in present tense and becomes ‘were’ for all forms in past tense.
Use
Subjunctive is used in some fixed phrases (formulaic subjunctive).

• God save the Queen!
  
• Long live the King!
  
• Thy will be done.
  
• Be that as it may.

Certain verbs (e.g. demand, insist, recommend, suggest) and adjectives (e.g. essential, important, vital) are followed by ‘that’ + subjunctive (mandative subjunctive) to indicate that something must be done (directive aspect).