One should be very careful while using तूने or तूas it should only be used in your closed friend circle of the same age group or if the other person is younger than you.
If you use it with strangers or in formal setting, you will offend the other person and embarras those around you.
However, as long as you are in the novice stage, I don't think people will mind, they will certainly be shocked though
Yes, it is because of the past tense and is a particle added to the subject to indicate the past for transitive verbs - those that take an object - as far as my understanding goes. That said, it would be helpful it a native speaker would enlarge on this and provide some clear examples, wouldn't it?
Found this thanks to another comment: http://www.learning-hindi.com/post/2392152114/lesson-83-the-past-tense-for-transitive-verbs
For Past Tense sentences with Transitive verbs the verb agrees with the object of the sentence and not, as we’ve been used to so far, the subject.
That is a cross cultural way of thinking about it. When a word is associated with a preposition - in this case, the subject in an English sentence - it becomes a 'circumstance' such that, the 'object' in English then IS the subject of the sentence. Hence, the concordance makes perfect sense, doesn't it? Just enlarging on your point...
That is a good question. The best I have come up with is to 'learn English' by registering as a Hindi speaker in Duolingo. The course is more developed than the present one. Whatever else it will add to your perspective and insights in terms of being a HIndi speaker. The downside is that you will hear more English than Hindi. Whatever else keep at it. It is quite a mind expanding adventure:-)
These past tense lessons are not well conceived. They are playing fast and loose with the English definite article. Sometimes it is used in the Duolingo translations and sometimes not, with no context for choosing. Well and good, but sometimes it is accepted in the Hindi translation and sometimes not, with no context for choosing. I just have to memorize Duolingo's required answer blindly, as it teaches me nothing about making the choice.