FEMALE PROFESSOR: But as I said, "fresco" means "fresh," and that's because to create a fresco, the paint has to be applied very soon after the plaster's 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+一个句子一起做主语