I came across an old friend in town last week.

came 是动词 come 的过去式，表示"来、到达"的动作。 across 是介词，本义是"横过、穿过"，引申为"在...的另一边"或"偶然遇到"。 came across 是一个短语动词，意思是"偶然遇见"或"偶然发现"。