【答案解析】There are two possible meanings in respect of this sentence. One possible interpretation of its grammatical structure is illustrated like this: (S I (VP saw (NP him (PP on the bus)))). This shows it meaning as: when I saw him, he was on the bus. Another possible grammatical structure could be illustrated in this way: (S I (VP saw him (PP on the bus))). This makes the meaning clear that when I was on the bus I saw him.