he has bought an australian car and has gone to alice springs时态问题以及其他几句

he has bought an australian car and has gone to alice springs时态问题以及其他几句
我不知道一般说话时候,什么情况用过去完成时,什么情况用过去式,新概念第二册中有一些问题请教大家：
1.he has bought an australian car and has gone to alice springs.buy不是瞬间动词吗,这里为什么要用过去完成时呢?为什么不用he bought an australian car?
2.I have just received a letter from my brother,Tim.He is in australian.He has been there for 6months.
同样搞不清楚什么情况用完成时或过去式,为什么不用i just received a letter from my brother?
接下来一句更不懂了,既然he IS IN australian.为什么要用he has been there for 6 months?他既然现在在澳大利亚,为什么后面一句又用完成时,他去了澳大利亚6个月?
3.last summer,i went to italy.我知道这里有last summer所以要用过去式.但是一般这种去了回来了不是应该用i've been to italy吗?如果用过去式就是强调去的动作吗?如果这里没有last summer,用过去式会不会奇怪呢?
he has been there for 6 months为什么不用he has gone there for 6 months呢,have been不是去了回来了吗?我们老师说gone表示去了没回来.