Finally, and often forgotten, the most strictly traditional sonnets should have closed syntax throughout. By “closed syntax” we mean two related things.
First, and more important, that the sonnet is usually composed of actual sentences in standard English; the sonnet form does not lend itself to the sorts of fragmentary or merely associative utterance often found in free verse effusions or more experimental verse forms.
Second, and more precisely the meaning of the term, in “closed syntax” the line boundaries are also phrase boundaries. This does not mean that line boundaries are necessarily sentence boundaries, though it is conventional for each quatrain to begin and end on a sentence boundary. But whether it is a prepositional phrase such as “into the applecart” or even a noun phrase such as “the sun that bakes the asphalt road ahead,” the line break should come before or after the phrase, not in the middle of it.
Breaking a grammatical unit with a line break is a technique called “enjambment”; the most strictly traditional sonnets will use it sparingly, if at all. Enjambment is typically used to indicate an overflow of emotion or thought (as though it could not be contained within the line), or as one way to set up a “false syntactic closure” effect (where the first words of one line incorporate the last words of the previous line into a larger grammatical unit that changes the apparent sense of the first line). Sonnet writers vary significantly in their observance of this convention, but it always exercises some degree of force; and the stricter the sonnet, the greater the power of any particular enjambment will be.