Normally when describing a noun, the adjective comes after it: La table brune. Irregular adjectives are adjectives that come before the noun it is describing: Une jeune fille.
To help you remember all the irregular adjectives that come before the noun, think of B.A.G.S.
B- Beauty A- Age Joli- pretty Jeune- young Moche- ugly Vieux- old S- Size (Except “grand” G- Good or Bad for people) Gentil- gentle Énorme- huge Mauvais- bad Miniscules- tiny
When using grand in terms of “great”, it comes after the adjective: Un homme grand. (A great man.)
When using grand in terms of “tall”, it comes before the adjective: Un grand homme. (A tall man.) All of this only applies to people.