They are of similar word class and grammatical form. But their non-interrogative usages differ because of syntactic constraints.
而非疑问用法由于受到不同的句式制约，产生了不同的表达方式。
No syntactic constraints are imposed on the ways in which expressions can be composed (though the language does have some semantic constraints).
虽然语言具有语义约束，但对于表达式的构造方式却没有施加语义约束。
However, certain semantic constraints can be expressed as syntactic ones.
然而，某些语义约束可以表达为语法约束。
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