Syntax refers to sentence structure and, specifically, the order in which words come in a sentence. Therefore, we might talk about regular syntax, what is a typical word order in a given language, or irregular syntax, what is an atypical word order. e.e. cummings is known for his irregular syntax because it is such a consistent feature of his poems. You may notice, for example, that this entire poem is actually only one sentence; it is quite complex, with semicolons and colons, independent and dependent clauses, but it is one sentence nonetheless. Typical sentences are infrequently this long or complex, and that alone makes its syntax unusual. However, in the third stanza, the narrator says,

or if your wish be to close me,i and
my life will shut very beautifully,suddenly,
as when the heart of this flower imagines
the snow carefully everywhere descending . . .

A more typical syntax would be to say that the snow is descending carefully everywhere rather than carefully everywhere descending because carefully is an adverb that describes the verb descending; in cummings's text, however, we get the modifier before we get the word it modifes. By inverting typical word order patterns, cummings draws more attention to the language he's using and adds a freshness to his work.

Syntax refers to word order in a sentence or clause or phrase.  Cummings is famous for his variations on “normal” syntax.  Every language has its own “rules” or “habits” of syntax; for example, in English an adjective usually goes before the noun it modifies (white wine), but in French the modifier follows the noun (vin blanc).  The same is true of adverbs and verbs – “normative” syntax puts the adverb after the verb (“moves gracefully”).  In Cummings’ poem, an example of “normative” syntax might be “small hands,” while Cummings’ variation of syntax might be “open always” (adverb before verb).  Beyond these small variations, Cummings also uses unusual sentence syntax, as in:

   you open always petal by petal myself as Spring opens   (touching skilfully,mysteriously)her first rose

These variations offer freshness to his texts, causing the reader to read more carefully and to savor his metaphors.  Add to this his unique use of punctuation and spacing, and his poetic signature is evident everywhere in this beautiful love poem.