In the mobile app there is a Russian word «неисчисляемые» in the tiles, but this is to select English words. I've already reported it, but the moderators haven't fixed it yet so I'm also adding this comment here
It's been appearing in almost all the exercises where you need to select from a list of English words. Hopefully more of us talking about it will bring more attention to it
When does it leave? must be accepted, as он can refer to a train, for example
I understood the usage of the prefix у on the word уходить, but does it happen often with other verbs too? And with other prefixes?
Yes, it's been a long time since I did Russian at school but I recall that there is a whole host of prefixes which can be put onto pretty much all of the verbs of movement. Have a look a little down the page on https://www.alphadictionary.com/rusgrammar/vom.html
Just a few questions ago, I used the present simple for this verb and was marked wrong for not using the future; so he I used the future and got marked wrong for not using the present simple.