本句是一个简单句 所以没有主句从句之分
it 是形式主语
was是系动词
the task of the grammarian是表语 后面 是不定式 短语 作 真主语
not simply to record alternatives,but to pronounce judgment upon them
其中 not simply.but = not only.but also 并列两个 动词不定式短语 作真主语
原句= not simply to record alternatives but to pronounce judgment upon them was the task of the grammarian