Masculin - Féminin Ecoutez

The feminine forms of some adjectives are based on an archaic masculine form.  This archaic masculine form is still used before masculine nouns that begin with a vowel or silent 'h'.

Masculine	beau	nouveau	vieux	fou	mou
Archaic Masculine	bel	nouvel	vieil	fol	mol
Feminine	belle	nouvelle	vieille	folle	molle
	beautiful	new	old	crazy	soft

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Normally, adjectives ending in -x replace the -x with -se to form the feminine. 

Masculine	Feminine
amoureux	amoureuse	in love
courageux	courageuse	courageous
curieux	curieuse	curious
ennuyeux	ennuyeuse	boring

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Masculine	Feminine
actif	active	active
bref	brève	brief

acteur	actrice	acting
conservateur	conservatrice	conservative
créateur	créatrice	creative

in:  http://www.languageguide.org/francais/grammar/adjectives/odd.html