so it is often difficult to tell whether they are boys or girls.中的difficult to tell whether是什么

so,连词,“所以”.
it is often difficult 是主句,it 作形式主语,
to tell whether they are boys or girls 动词不定式作真正的主语,而 tell 后面的 whether 引导一个宾语从句.
原句中文意思：所以经常难以判断他们是男孩还是女孩.