There are a lot of rules in any language, but French seems to have more than its fair share. Here are some things you should know when agreeing adjectives to gender

Most adjectives can be made feminine by adding an e to the masculine form.

grand = grande
If the masculine form already has a mute e, there is no change for the feminine version of the adjective.
optimiste = optimiste
Lots of groups of adjectives follow particular rules (of course). There are several exceptions, but the rules hold true in many cases.
  • If the adjective ends with an x in the masculine form, simply replace the x with se to make that adjective feminine. For example: studieux = studieuse

    The exceptions are faux (fausse), roux (rousse), and doux (douce).

  • If the adjective ends with er in the masculine form, replace the er with ère to make that adjective feminine. For example: premier = première

  • If the masculine ending is et, replace the et with ète to form the feminine adjective. For example: complet = complète.

    The exceptions are muet (muette) and coquet (coquette)

  • If the masculine ending is f, replace the f with ve to make the adjective feminine. For example: actif = active

  • If the masculine ending is il, el or eil, add an le to the end of the adjective to make it feminine. For example: gentil = gentille, spirituel = spirituelle, and pareil = pareille

  • If the masculine ending is en or on. add an ne to the end of the adjective to make it feminine. For example: ancien = ancienne, and bon = bonne

  • If the masculine ending is c, replace the c with che to make the adjective feminine. For example: blanc = blanche

    The exceptions are : public (publique) and grec (grecque)

  • If the masculine ending is g, replace the g with gue to make the adjective feminine. For example: long = longue

  • If the masculine ending is eur and the adjective is derived directly from the verb, replace the eur with euse to make the adjective feminine. For example: travailleur (verb is travailler) = travailleuse

  • If the masuline ending is eur and the adjective is NOT derivd directly from the verb, replace the eur with rice to make the adjective feminine. For example: conservateur (verb is conserver) = conservatrice

  • If the masculine ending is eur and it is an adjective of comparison, replace the eur with eure to make the adjective feminine. For example: meilleur = meilleure

There are some adjectives that follow no rules and have to be memorized by themselves. Everyone loves these guys:

Épais = Épaisse
Favori = Favorite
Frais = Fraîche
Gros = Grosse
Beau = Belle
Nouveau = Nouvelle
Vieux = Vieille

To complicate things further, beau, nouveau and vieux have alterate masculine forms that have to be used with singular masculine nouns beginning with a vowel or mute h.

un beau manteau vs. un bel imperméable
un nouveau canapé vs. un nouvel appatement
un vieux monsieur vs. un vieil homme

Need help? Let me know!

Most of this information came from
my brain and was reinfoced by a review
sheet I recieved from a professor.