According to Biber et al. (2003), why are semi-modals like have to or be going to considered more structurally "flexible" than central modals?

Question

According to Biber et al. (2003), why are  semi-modals  like  have to  or  be going to  considered more structurally "flexible" than central modals?

Accepted Answer

They can shift for tense, show person agreement, and link with other auxiliaries.

Answer

They completely reject the infinitive particle  to .

Answer

They are restricted exclusively to written registers.

Answer

They can only be used in highly formal academic discourse.

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