A doll is being made by Mary.
这句话是现在进行时态+被动语态.
它等价于：
Mary is making a doll.
这句话是现在进行时态+主动语态.
原句因为要表达被动含义所以用is made,又要用进行时态,所以就用is being made.
be是谓语动词,being不是谓语动词（现在分词是非谓语动词）.
A doll is made by mary.是一般现在时,和原句的时态不同.
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