When praying jamaah men and women are separated by a curtain, but before start praying the women open the curtain fully and I dont know for why until we can see them. The last row for the men is in front the girl first row and we can see them. Is it permissible like that? I dont understand why need to open the curtain.

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I think they are opening the curtain because they believe that Jama'ah is not correct if there is a barrier between men and women, which is not true.
Jama'ah is connected even if there is a wall between them, as long there is distance of 1 meter between the last raw of men and the first raw of women.
Hence, in your case, there is no need to open the curtain.
However, opening the curtain does not effect the jama'ah, but the men should not look to women.