red前面用a。an只有发元音的单词前面才用。a red的例句：She wore a thick tartan skirt and a red cashmere sweater（她穿了一条厚厚的格子呢裙和一件红色羊绒衫）。

a red的例句：She was wearing a red flower in her hair（她头上戴了一朵红花）；Beijing is indicated on the map by a red star（在这幅地图上北京是用一颗红星标明的）。