Sentences with Simple Transformations are in Stage 3; while Embedded Sentences and Relative Clauses appear in Stage 4.
In Stage 3 the MLU ranges between 2.5 and 3.0 morphemes. Children's ages range from 2 1/2 years to not quite 3 years (34 mos.)
Simple sentences with both noun and verb phrases are the unit now. Some basic transformations are also evident, like the "Yes/No" Question Transformation, and Negations.
Towards the end, some auxiliary forms appear in questions.
These may include words like "be," "can," "will," and "do."
In Stage 4 the MLU ranges between 3.0 and 3.75 morphemes. Children's ages range from almost 3 years (35 mos.) to not quite 3 1/2 years (40 mos.) A major transformation here involves embedding one sentence in another, and the use of relative clauses. For example, "There is the boy who hit me."