I think this should be "MIt csinálsz?" or "Mit csinálsz most?"
"Mit csinálsz te?" is what you typically say to a child when they're doing something wrong, like drawing on the walls. :)
Mi is the nominative case, while mit is the accusative case. The reason you use the accusative case is because we're talking about the object of "doing," which is represented by "what," hence "mit".
It's rather interesting how the word "csináls" resembles the dialectic and archaic verb form in the 2nd p. Sng. Pres. „чиниш“ /'tʃɪnɪʃ/. Makes you wonder about etymology.
I didn't exactly know where to ask this so I'll just ask this here: one of the sentences I've come across were the sentences "egy lány van itt" and "a szék mindig itt van". According to my Hungarian mother "a szék mindig van itt" is wrong, but why is "egy lány van itt" correct? What exactly is the rule behind this? I'm sorry if this is explained later on, but I've just been wondering
A szék mindig van itt. is indeed incorrect and Egy lány van itt. is correct
It is because of the word mindig. This word and a few more has a compulsory word order
Mindig itt van.
Néha itt van
Állandóan itt van
Ritkán van itt.
Soha sincs itt.
So, A lány van itt. becomes A lány mindig itt van.
A szék van itt. will be A szék mindig itt van.
My hungarian frend told me that they say "te mit csináls?" rather than "mit csináls te?". Is it everywhere like that or what?
Both are correct. In the first case, the stress is on the person, the second case want to know about the action more.
"Mit csinál ma" would mean "What are you/he/she/it do today?" Csinál can be a formal second person singular conjugation, as well as the third person singular conjugation. Csinálsz is the second person singular, informal version.
Why is 'what are you making' incorrect? I thought it could be either doing or making?
It's a good translation. Though "What are you doing?" is far more common to ask.