she is sure to be successful为什么用be be后面接什么词 she wants to be a doctor 我不是太不清楚But as they soon discovered ,disguises can sometimes be too perfect.这个be 又是怎么回事？

因为be表示状态，to后面接动词原形，所以用be原型
be后面接 adj 或 n或者是介词短语

to do 是动词不定式，to 后接动词原形啊，所以用BE,
be 是系动词，后面接形容词或者介词短语或者名词构成系表结构
例如 He is happy/He is at home/He is a student

be在这里是“成为,当...”的意思,be sure to...是“一定会...,肯定会...”表示肯定的语气.比如Lily is sure to make a mistake.Lily一定会犯错.Tom was sure to be a good husband.Tom一定会成为一个好丈夫.
be successful 又是另外一个短语,是成功的意思.比如说She is successful,她很成功.
be sure to be successful相当于是而这的结合,就是“肯定会成功”的意思.这样解释行吗
But as they soon discovered ,disguises can sometimes be too perfect.
看来你是没有明白.
用形容词来修饰名词,得用be动词连接,这个应该知道吧,比如she is beautiful the candies are sweet
同样,我们一般说disguises are perfect 假象很完美/完美的假象 但是在系动词如can,could,shall,should,must 等待后面接的be动词得以原形的形式出现.比如she is late她迟到了/she must be late.她一定迟到了.
懂了吗

Be是连系动词，其后面可以跟作表语成分的形容词，介词或介词短语，to do或donging等非谓语动词和作宾语成分的名词等。