Hello All, do you happen to know an app /website that cam help me this way? I know some words and some grammar already, and now I feel nesessity in practicing of making sentences. Is it possible that the app, knows what I already studied, will throw at me random words and ask me to put them according to certain sentence structure? I don't care if some of this won't make senes (it makes exercise even more funny), only practice matters.
It's not randomly generated like you're describing, but freeform sentence writing practice exists in Renshuu in several forms already.
When you learn a new piece of grammar, you can go to the Grammar Library and submit your own sentence using it.
The Word Garden in the Community menu provides a list of terms and you can submit a sentence using those.
Also in the Community menu is Question Corner, which asks a question you can answer using whatever grammar and vocab you want to challenge yourself with.
Regular grammar and sentence schedules also acheive something similar to what you're asking for with a set correct answer, if you enable jumble-style questions which make you correctly arrange the words and grammar points in the sentence. If you follow the Renshuu Japanese Basics curriculum, all of the grammar questions are designed to incorporate words you're already learning.