Why not "Já čtu teď tu velkou knihu" ? Duo refused it. THX.
The word order is less common but it is acceptable.
Why is this word order wrong? Ja čtu velkou knihu teď
You need "tu velkou knihu". The word order is possible and stresses that you are reading it NOW.
Why not "Čtem teď tu velkou knihu"?
čtem is Slovak, not Czech. In colloquial Czech it can be plural, not singular.
Is this wrong?"Teď tu velkou knihu čtu?"
Yes, it is very strange. For "I AM reading a big book now!" (meaning: you are not, but I AM reading such a book) I can imagine "Já teď velkou knihu ČTU!" but without the Já pronoun it is very strange.