第1句
I'm not thirsty because I drank a lot of water this morning. 感觉上是指现在不渴，因为早上喝过水了(过去时)
第2句
She started to go to school at the age of five. She was the youngest pupil in her class at that time. 不知道是否学过at the age of 词组，但要比状语从句when 简单。
第3句
Bill played the computer games for three hours, so he didn't finish his homework. 电子游戏用复数
第4句
He was on time for the school bus, so he was not late. 因为已经讲到了赶上校车，后半句上学没迟到for school
可以省略。
第5句
I was born in China but grew up in America, yet I will love my homeland forever. 出生在中国和在美国长大是转折非并列，不能用and..

如果用现在完成时，表示过去的动作造成现在没有完成作业。
Bill has played the computer games for three hours, so he doesn't finish his homework.
并非遇见 for +一段时间都要用现在完成时或过去完成时的。
比如 Bake the cake for 40 minutes.  They stopped for a short time to rest the horses. (Longman Dictionary of Contemporary English)
呵呵 英语语法还是很复杂的。这个for +一段时间的用法是要根据上下文的。不过对小学生和初中生来说太深了。.