My dear Gerd, let me talk about your cruel grammar exercises. Anyway, I think they are quite useless, but I would give you some small hints to handle them. You handled them as a mathematical exercise, not as a language exercise. It’s a big mistake to use the language as a mathematical model which obeys grammar rules and so on. If you like to solve such mathematical problems you should better take them as a language exercise and I would recommend you not to fill out gaps. Filling out gaps will not help you to memorize your vocabulary, the structure of the sentences and so on. Write down the original sentence, and during writing down, think about what is the object, not the grammatical object, the object which is to be handled, and change your perspective from the actor of the action to the object of the action. And tell yourself how to focus your utterance on the object and not anymore on the actor. And so you have a feeling for paraphrasing the sentence on the basis of the meaning of what you want to say. And in this way you will find the right transformation automatically. I mean, your English language level is much too high for solving such ridiculous and senseless tasks. But, for instance, paraphrasing and writing down the paraphrase is always, for any kind of learning languages, the best way to make your memory stronger and you can recapitulate the structure of the language.
Coming to the end, I like to tell: don’t be afraid speaking English with me. It’s a good chance pushing up self-confidence in speaking English and I think my English isn’t so bad that I can worse, reduce, diminish yours. You know I’ve a lot of opportunities of English conversation, nevertheless I’d like to speak with you also once a week half an hour. Think about it! See you!