Stage 5 brings Coordinated Conjoined Sentences and Propositional Relations.
In Stage 5 the MLU ranges between 3.75 and 4.50 morphemes. Children's ages range from almost 3 1/2 years (40 mos.) to not quite 4 years (46 mos.) Complex sentence transformations include coordinated conjoined sentences and propositional relations. For example, "He ran away because he hit me." It couldn't be, "He hit me because he ran away."
More Modals, more consistency in auxiliary inversions, and some adult level tag questions are just a few of the complexities added to the language system.
Whereas the rules of morphology deal with additions, subtractions and transformations to or within words, Syntax deals with these manipulations at the sentence level.