Special Uses of the French Subjunctive:Usages particuliers du subjonctif

Although the French subjunctive is most often used in subordinate clauses (e.g., Je veux que tu fasses), it can also be used in main or independent clauses.* In general, this usage tends to be formal.

Que + subjunctive This construction is used for third person commands, wishes, concessions, suppositions, and exclamations.
	Que Dieu bénisse l'Amérique.	(May) God bless America.
	Qu'ils mangent de la brioche !	Let them eat brioche!
	Qu'elle finisse avant midi !	Let her finish before noon! I hope she finishes before noon!
	Que le bonheur vous sourie.	May happiness smile upon you.
	Qu'il ait menti : il ne faut pas le punir.	He may have lied / It may be that he lied, but he shouldn't be punished.

The subjunctive is also used with certain verbs to indicate polite and/or literary commands, wishes, etc.
être	Soit un rectangle ABCD.	Let ABCD be a rectangle, Let's take the rectangle ABCD.	science, mathematics
pouvoir	Puissiez-vous ne jamais voir une telle chose !	May you never see such a thing!	literary
savoir	Je ne sache pas que cela existe.	I'm not aware that that exists, As far as I know, that doesn't exist.
savoir	Pas que je sache.	Not that I know of.	literary
venir	Vienne l'aube, les oiseaux chantent.	Come dawn, the birds sing.	poetic, literary
vivre	Vive la France !	Long live France!	third person command
vouloir	Veuillez m'excuser de vous déranger.	Please excuse me for interrupting you.	very polite command

Finally, there are a few fixed expressions which take the subjunctive.
	Ainsi soit-il	So be it
	Advienne qui pourra	Come what may
	coûte que coûte	at all costs
	Dieu soit loué !	Thank goodness!
	Dieu vous protège.	May God protect you.
	Grand bien vous fasse !	You're welcome to it! May it serve you well! (sarcastic)
	n'en déplaise à...	with all due respect to...
	Qu'à cela ne tienne.	Never mind that; That doesn't matter.
	Sauve qui peut !	Save yourselves (if you can)!

*Note: For que + subjunctive and many of the fixed expressions, the subjunctive is needed because the original main clause has been dropped, leaving what used to be a subordinate clause to stand alone.