Sentence:  Using the same type of distributional criterion, we could argue that only a Verb (in its "base" form) can occur in the position marked — in (23) below to complete the sentence:/ (23)     They/it can —/ [...]/ Conversely, the only type of word which could be used to begin a three-word sentence such as (25) below:/ (25)     — I be frank?/ is a <WSD>Modal</WSD>: cf. [...] 
Meaning of the word marked with <WSD> tags:  modal verb