I was on the pavement next to my house and I was calling to my friend, when suddenly I saw a man who [was] just going around to my neighbour's car, which was parked opposite.

was 是 be 动词的过去式，表示过去某个时间点正在发生的状态或动作。 opposite 是形容词，意思是"对面的、相反的"，指位置或方向相对的另一侧。在这里指停在马路对面的车。