Silesian

I just discovered something amazing

If compare sentences from Polish and Silesian "I am a student"

Polish :1. Ja jestem studentem1a.
IP[NP Ja[ I -em[ Vp byc [ NP-nom. studentem]]]
We can drop the NP "ja" from the spec of IP and substitute it with the PRO
1c. jestem studentem
IP[NP PRO[ I -em[ VP byc[ NP-nom. studentem]]]
The sentence is still grammatical.

Silesian:
2. Jo zech je sztudyntym
2a. IP[NP jo[ I zech[ VP je [NP-nom. sztudyntym]]]
3. Joch je sztudyntym
3a. IP[NP jo[ I -ch[ VP je[ NP-nom. sztudyntym]]]
4. Jo je sztudynt
4a. IP[NP jo[ I [ VP je [NP-nom. sztudynt]]]

However in Silesian we cannot drop NP from the spec of IP substytuting it with the PRO in all cases:
2c. * zech je sztudynt
2d. IP[NP PRO[ I zech[ VP je [ NP-nom. sztudynt]]]
3c. * je sztudynt
3d. IP[NP[I -ch [VP je[ NP-nom. sztudynt]]]
4c. * je sztudynt
4d. IP[NP PRO[ VP je[ NP-nom. sztudynt]]]
However a sentence
5. Zech je sztudynt
IP[NP PRO[ I zech[VP je[ NP-nom. sztudynt]]]
is totally acceptable and grammaticalWe can also reverse it and say:
5a. Jezech sztudynt
IP[NP PRO[I zech[ VP je[ NP-nom. sztudynt]]]