Snap decisions can be important defense mechanisms,这句can be是

Snap decisions can be important defense mechanisms,这句can be是什么结构
Snap decisions can be important defense mechanisms中,can be是什么结构?我看decisions是动词第三人称单数,请问有do+can+be这样的结构吗?can be在句子中用作什么语法成分?
Snap decisions can be important defensemechanisms; if we are judging whether someone is dangerous,our brains andbodies are hard-wired to react very quickly,within milliseconds.But we needmore time to assess other factors.
随便介绍一下整句的语法结构。呵呵