一个名词+一个句子 可以做主语？

FEMALE PROFESSOR:But as I said, fresco means fresh, and thats because to create a fresco, the paint has to be applied very soon after the plasters been spread over a surface, right onto the wet, fresh plaster.

FEMALE PROFESSOR:By doing this, the painting actually becomes part of the plaster.Finishing a painting before the plaster dry(or dried 不确定） was a real challenge for fresco painters.

请问，老师这句话的主语是什么？ 我的理解是 before 是conj, dry是不及物动词，Finishing a painting before the plaster dry 这种一个名词+一个句子一块做主语

before the plaster dry作 finish 的状语，修饰finish这个动作的完成时间。

“Finishing a painting before the plaster dry”整个做句子的主语。

dry是before引导的时间状语的谓语。