Adjectives are words which modify or describe nouns. Since they tell us about a noun, they must agree in number and gender with the noun they are describing.
Numbers and adjectives which tell how many, precede the nouns they modify:
tre arance = three oranges
molti libri = many books
troppo rumore = too much noise
Adjectives of color always follow the nouns they modify:
una casa rossa = a red house
una penna gialla = a yellow pen
Most other adjectives will follow the nouns they modify, but you will learn through conversation that some are flexible and may come either before or after the noun:
una donna intelligente = an intelligent woman
un uomo gentile = a kind man
Adjectives ending in 'o' agree in both number and gender with the noun they modify. Adjectives ending in 'e' only agree in number:
ragazzo alto = tall boy
ragazzi alti = tall boys
ragazza alta = tall girl
ragazze alte = tall girls
uomo forte = strong man
uomini forti = strong men
donna forte = strong woman
donne forti = strong women