Were it not for the professor's crucial letter of recommendation, his application would never have been accepted.

"Were it not for" 是正式语体中的倒装虚拟条件结构，相当于 "If it were not for"，引出与过去事实相反的假设条件；后接名词短语 the professor's crucial letter of recommendation 作为条件内容，主句用 would never have been accepted 表示在反事实条件下的结果推断。