Не думаю, что у этого есть какие то правила или законы. Это одна из тех вещей, которые лучше просто учить. Это как irregular verbs в английском. Чем больше ты будешь их использовать, тем лучше запомнишь. К примеру, есть "сладкий-слаще" и это немного выбивается из твоего списка. Так что, не пытайся структурировать это, а просто запоминай каждое отдельное слово. Их, на самом деле, не так много, так что не бойся ;)

Русских в италки действительно много)  but I also can help you. Call me in Skype if you want to have a talk or write if you want to chat) 

Some schools in my country Russia really teach economic, but in my school we haven't learned it. We usually don't think that it's important, because even if children have an economic in their school they usually study it in home with parents

You're interesting )  I'm not in a Paris, but if one day you will come to Russia, write me and I'll can to help ya, for example to operate an excursion in Moscow)) 😉

my Japanese tutor usually sends me some examples of conversations or audio materials for our manual. Also we talk usually, too. And we watch videos in the YouTube in Japanese. And she told me to download italki, because she isn't a native speaker and when I talk with her I usually use Russian (cause I'm Russian) to explain something. So it will be better to have a talk with some natives. Maybe you have some friends that can have a chat with your students in the lesson? p. s. oh, I have a new idea. how about practising with Google Translate? DD I mean, your students'll need to talk correctly if they do it with a bot :))

Hey, I see, youre making friends here) can I introduce myself too?) I'm Kate from Russia, learning English and Japanese. Im going to study in Japan when I finish my studying in college in here) write me whenever you want to) also, this is my Skype : katja556g