There is a grass stain on his jeans after playing football.

stain 是名词，意思是"污渍、斑点"，指衣物或物体表面留下的难以去除的痕迹。这个词也可以作动词，表示"弄脏、染色"。 grass stain 特指由青草汁液在衣物上留下的绿色污渍，通常较难清洗干净。