He buried himself into study just now as if not knowing what had happened around.

He buried himself into study just now as if not knowing what had happened around.

严格说这句话有个小错。
当连词as if 引导的从句之主语与主句主语为同一人物，且谓语动词包含be动词时，主语和be动词可以省略。如果我们恢复省略部分，完整的句子是：
He buried himself into study just now as if he was not knowing what had happened around.
但是，know这一动词为非持续性动词，是不可以有进行时态的，所以这句话是错误的，因此也不可能省略了he was。正确的说法是：
He buried himself into study just now as if he didn't know what had happened around.
那么什么情况下可以省略呢？以下句子中的be动词都使用正确，因此就可以省略主语和be动词：
He rests his chin on his hand as if (he is) thinking about something. (进行时态）
She lay silently as (she was) asleep. (be+形容词表语）
The boy stood motionless on the floor as if (he was) frozen. （被动语态）
可以这样省略的连词还有when, while, though, if, unless.
希望你明白了我的解释。

刚才他埋头学习好像不知道周围发生了什么追问语法有没有问题

句子没有问题。as if not knowing。。。 是带连词的现在分词短语作状语相当于as if 引导的状语从句作状语。
不过该短语中的宾语从句最好用过去进行时，才能体现主句物我两忘的学习时的境界