1）Half an hour went by,we had to walk home because the last bus had not come.句子主干：我们只好走回家 “we had to walk home”（时态可用一般过去时或现在时）        “末班车还没有来”可以作为主干句子的原因：because the last bus had not come          时间状语为 半个小时过去了half an hour went by

（2）Mary seems to be worried about the Chinese exam because she has not learnt the text by heart.句子主干：玛丽看上去对汉语考试很担心“seem to”看上去“be worried about”对…担心；原因状语从句：因为她还没有背熟课文（句子时态用现在时更贴切，而“背熟课文”这个动作在“担心”之前持续到现在还未完成，用现在完成时）

（3）Since the basketball game has been postponed, we might just as well visit the museum.句子主干：我们不妨去参观博物馆。“不妨”might/may just as well, “既然篮球赛已经被推迟”可做原因或条件状语从句（篮球赛被推迟应用被动语态be postponed）

（4）He lived with his parents all the way in Australia through World War II句子主干：他一直和父母住在澳大利亚。“一直”all the way。时间状语：整个二次世界大战期间through/during the World War II. 发生在过去，整个句子用过去时。

（5）I can say that I kind of have lost the touch with my college classmates since I graduated from Nan jing University in 1985句子主干：我与我的大学同学失去了联系（失去了联系“lose touch with…”）时间状语可用since引导的从句（可放在句首或句末），从…毕业“graduate from”，毕业发生在1985年，是过去时。从句用一般过去时，主句用现在完成时。