Which explains why the structure shifts to stanzas of 3 lines for the final three stanzas

modal adverbs are used to modify specific verbs that consist of a linking verb (verb of being) and sometimes another verb. examples of modal verbs include can, could, may, might, must, shall, should, will and would.

modal auxiliary. an auxiliary verb characteristically used with other verbs to express mood, aspect, or tense. in english, the most common modal auxiliaries are can, could, may, might, must, ought, shall, should, will, and would. also called modal .