"So what is the child doing motorically at 2 to 2 1/2 years, during the stage of Syntactic Speech?
“Daddy (actor) Ball (object),”; or,
“Daddy (actor) throw (action);” or
“Throw (action) ball (object).” All of which may mean, "Daddy throw the Ball.”
The human brain seems genetically predisposed to search out these noun phrase-verb phrase rules for ordering of these semantic features in a two word sentence.
From here, the development path leads to three and four word more complex sentences as described by Brown. This is our inheritance and a prerequisite to language development.
Motorically, babies can walk now with ease, and even run; and can manage going up and down the stairs, putting both feet on each step at a time.