My parents went to Ireland for two weeks, and I was responsible for the house.

was 是 be 动词的过去式，表示过去的状态。 responsible 是形容词，意思是"有责任的、负责的"，来自名词 responsibility（责任）。 for 是介词，在这里表示"对于、关于"的意思。 was responsible for 是一个固定搭配，表示"对...负责"或"负责照料..."的意思。