(S (S (PP (BASENP (NN ４) (NN 月) (NN ２２) (NN 日)) (IN に)) (COMMA 、) (S (PP (NP (PP (BASENP (NN 湾)) (IN の)) (NN 表面)) (IN に)) (S (PP-SBJ (BASENP (NN 油膜)) (IN が)) (VP (VBV 現れ) (VP (MD た)))))) (PERIOD 。))
4 月 22 日 に 、 湾 の 表面 に 油膜 が 現れ た 。
On April 22 , an oil slick appeared on the surface of the Gulf .