I did not know his nephew Jack became a leatherneck.是什么意思？

I did not know his nephew Jack became a leatherneck.

我不知道他的侄子杰克已加入海军陆战队.

did

v. 做( do的过去式)，干，学习，研究；

例句
He did have a real knowledge of the country...
他对该国的确知之甚多。

not

adv. 不，[用以表示否定、否认、拒绝、禁止等]不是，几乎不，未必，没有[用于否定后面的词或短语]；

例句
It became very clear that the incident was not just an aberration, it was not just a single incident.
事实已经很清楚，这件事不是偶发的，它并不是一起孤立的事件。

know

v. 知道，了解，认识，确信；n. 知情；

例句
I know you're above me socially, but I must say I find your attitude offensive...
我明白你的社会地位比我高，但我得说我觉得你的态度很粗鲁。

his

pron. (he的所有格)他的，(he的物主代词)他的东西；det. 属于他的，用于名词前，自己的，指某人的，上帝的；

例句
He claimed that his parents had abandoned him...
他声称父母遗弃了他。

nephew

n. 侄子，外甥；

例句
She has a nephew who is just ten years of age...
她有一个才10岁的侄子。

jack

n. 千斤顶，[信]插孔，插座，男人，纸牌中的J；vt. 提高，增加，用千斤顶顶起，抬起，抬起，使想起；vi. <美>用篝灯打猎（或捕鱼）；

例句
He told her their affair would have to stop, because Jack Smith was blackmailing him...
他告诉她说他们的婚外情必须结束了，因为杰克·史密斯正在敲诈他。

became

v. 变为，成为（ become的过去式 ）；

例句
It became very clear that the incident was not just an aberration, it was not just a single incident.
事实已经很清楚，这件事不是偶发的，它并不是一起孤立的事件。

leatherneck

n. 海军陆战队员；