I am secretary of the City Student Union.
这种情况下指职位、头衔、身份不加冠词的原因是学生会秘书只有这个。
反之如果不只一个就要用.I am a secretary of the City Student Union.
你举的那个例子就是属于这种情况,老师不只一个。
此外,a secretary这个结构强调的是secretary这个词,
与其对应的是a junior member /a chairman等结构
也就是说你想告诉读者的是:你是秘书,不是其他的普通成员或者主席.
至于第二题为了用that的原因,我们就要从他们的用法上区别了.
that(those)指代具体的名词,他们指代的名词要有the限定
that指代单数名词(包括可数不可数名词的单数)
如 In winter,the weather in Beijing is much colder than that in Guangzhou .
The boy in the red is talled than that in the blue.
those指代复数可数名词.
如 The students in Class 4 are much more noisy than those in Class 5
我们可以发现that/those既可指人也可以指
而it指代的一般是事物,而通常指带不定时和动名词,
如 Leaving a rich life nowadays is much easier than it was 10 years ago.指代leaving a rich life
To swim in this river causes much more risk than it does in that river.指代 to swim in this river\
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