I heard the birds----(sing)in the forest just now 请问这里用sing

Singing.
两个都行,要看句子.hear sb do sth.意思是听到某人做某事,强调状态,强调此人一直在做事,而hear sb.doing sth.则强调此人正在做某事,强调动作.在这句话里,强调刚才听到鸟儿唱歌,强调唱歌的状态.