Well, Luca, This is William from China. I have to say you have issued a very good question. Cause chinese lacks of morphological changes in tense, the word order is very important to convey different grammatical meanings. 了 and 过 are 2 very important constuctional particles to define the tense. Here I only answer the questions you list, no more others 1* 我去了中国 verb+了+obj (maybe I'm still there, maybe not) 2* 我去中国了 verb+obj+了 (I'm still there) 3* 我去过中国 verb+过+obj (I went and came back) They are not the same definitely. And you are right in this point, but not complete. 6* 我去了看病 7* 我去看了病 8* 我去看病了 (I'm still with the doctor) Yes， 6 is wrong, but 7 is right in grammar. 8 is right in grammar.,but the english explaination is not. Mostly, if a sentence is right in grammar, it may have more than one meaning in tense. It depend on the contexts. That is why that Chinese told you that they are the same. Probably he does not care about it much, so there is little difference among them. I have to say, The Higher level you reach in Chinese, the more complex it becomes. Usually many native Chinese people are not clear about your questions, too. cause they never care about it, just express colloquially. I am sure you have achieved quite a high level in Chinese, so do not ask anyone on the street to clarify your confusions Thank you

First of all, for your question, between 6&7, 7 is right! 我去看了病 Let me put it this way: the former example you mentioned 我去了中国 and 我去中国了 are both right; but between 我去了看病 and 我去看了病, only the 我去看了病 is correct. The difference lies on the word right after 了 here, 看 from 看病 is a verb, you cant connect two different verbs with 了 in the middle(except: 看了看/听了听/试了试 ect,, the first verb and the second verb are the same); however, you can put a noun right after 了, in this case, '我去了中国'---中国 is a noun. more examples for your understanding: verb+了+noun: (correct) 丢了钥匙。 我看了电影。 我做了作业。 我去了电影院。 verb1+了+verb2 (wrong) 我去了看电影。 我去了吃饭。 我去了上课。 Note1: you will notice my examples are all with 去 bla bla, this is because verbal phrases are often formed with this pattern 'go to do something ' in English too(go to do homework, go to travel,etc.) Note2: if you want to correct, there are two ways, one way is just omit '了‘ （我去看电影；我去吃饭）or put 了 behind the sentence, 我去看电影了；我去吃饭了。 The two patterns are correct grammatically, but have different emphasis. If i have time, i would love to talk about this more, but i gotta go to have a lesson now! Hope this makes sense to you! Good luck!

我可以负责任的说，你的老师讲的是错误的。 我去了中国。 这一定是你在中国之外说的话，绝对不可能是在中国之内讲的。 如果你在中国之内，你应该说，我来了中国。我到了中国。 比如，你身在中国，你和你的外国朋友交流（通过网络之类），朋友问你：上个月你突然消失了，你去哪了？你会回答“我去了中国“吗？因为你现在身在中国，你一定是回答，我来中国了。而不可能是”我去了中国“或“我去中国了”，”来“和”去”的用法你应该学习过了吧。 我去中国了。 一样不是在表达我现在依然在中国。（I'm still there.）你说这话时一定不在中国。道理很简单，如上。 就算这句话可以表达你在中国。但它同时也一定可以表达你不在中国。比如，你到过中国，现在回到了你的祖国，见到了朋友，朋友（当面）问你，你这段时间到哪里去了？，你怎么回答？难道只能说“我去了中国”吗？事实上，许多中国人会说，我去中国了。而不是“我去了中国”，因为对方问的是“你去哪里了”，我们回答这样的问题时会不自觉的使用与问题相同的结构：你变成我，去哪里变成去中国，加上了。而如果对方问：你这段时间去了哪里？我们会不自觉的回答：你-->我，去了，哪里-->中国，于是脱口而出，回答：我去了中国。 所以说，上面的两句话都表明，你不在中国。试问：你身在中国，为什么还要说“去”，“去”哪里，说明你本身不在那里，才会用去，也就是你说话的时候根本不在中国。而加了“了”也好，“过”也好，表达的是完成，过去之类的意思。 “了”，可以表达一个动作已经完成，也可以表达一个动作还在进行，这是肯定的，但“去”这个词很特别，加上一个特别的人称“我”，就无法表达“我依然在中国”这样的含义了。比如我们说：他去了中国，他去中国了。那么“他”现在就一定在中国，而不是在中国以外。而换成“我”，恰恰相反，我现在一定不在中国。 我去过中国，也不是I went and came back的意思，这句话更多的是在表达，我在过去某个时候到过中国，并不强调“我又回来了”。因为你说话的时候一定是不在中国的，因此是个不言自明的事，所需表达出来呢？ 去年我去过中国（了）。这是完全正确的汉语。我不知道你们老师是从哪里得出来的“过 should not be used with time words that define the time of happening”，完全是没有道理的。 时间本身就可以分为“今年、去年、明年”，我要表达“去年”我到过哪里“，请问”过“不能和”去年“这样的time words一起用，我应该如何表达我是去年去的，而不是今年，前年去的呢？”过“表达”过“的含义，”时间“词表达时间词的含义，各起各的作用。只有逻辑上重复的时候才可能不使用。 反过来，去年我去过中国，如果把”去年“去掉，请问，你是什么时候去的中国？一百年前吗？就是扯。可以肯定的说：你可以放心大胆的说”去年我去过中国“、”我去年去过中国“、”去年我去过中国了“这些句子都是正确的。 同样的道理：我去看病了。无法表达I'm still with the doctor.恰恰相反，你说话的时候，肯定没有和医生在一起。 不想说了。不放弃汉语有时态的观念，不放弃”着了过“是时态标志的观念，很难学好汉语。用西方语法体系理解汉语，难！

了is behind of verb or adj or verb obj 了isn`t in front of verb or verb obj 看is verb,看病 is verb obj so 6 is wrong

I think I can help you.Do you want to practice the Chinese?