Children under the age of 2 can understand the difference between two sentences in which the subject and predicate are reversed. For example:
Mommy is helping daddy.
Daddy is helping mommy.
By the time students enter the classroom in pre-K or kindergarten, most are able to give the correct answer when asked, “Which sounds better?” for this example:
Swims the duck.
The duck swims.
Students with solid sentence sense can recognize if a sentence is syntactically incorrect when they hear or read it. The level of syntactic awareness that students bring to kindergarten often depends on how much exposure to oral language they had prior to entering school.
Syntactic awareness, especially the ability to write effective academic sentences, should be taught to all students beginning in kindergarten. By third grade, most students have developed a basic understanding of the difference between a complete sentence and a sentence fragment. You should use the terms subject and predicate (rather than naming and action parts) by third grade.
Some students beyond third grade who have difficulty with sentence writing need continued instruction about sentence basics.