Adjective endings

When an adjective comes before a noun, it takes an ending. The ending depends on the article in front of it: if the article already shows the gender and case clearly (der, dem, den), the adjective just adds -e or -en. If the article doesn't (ein, or no article), the adjective takes over the job and shows it instead.

That's why it's der rote Wagen but ein roter Wagen: der already says 'masculine', ein doesn't, so the adjective adds -er. Learn three patterns in practice, not a forty-eight-cell table.