Semantic Roles provide a framework in Stage 1; and Phrase Structure, Inflections and Prepositions appear in Stage 2.
Semantic relations (roles) are clearly in evidence. For example, agent + action (Doggie run) or action + object (Throw ball), would be a typical expression. Intonation on a word is used to communicate questions.
In Stage 2 the MLU ranges between 2 and 2.5 morphemes. Children's ages range from about 2 and 1/4 years (27 mos) to 2 and 1/2 years (30 mos.) Mainly, phrases are the units of expression.
Noun and Verb Phrases are recognizable now. Some "wh" pronouns (like "where") are added to the beginning of a noun phrase.
And some verbs like "do," "go", and "read" etc. are appended to the beginning or the end of the phrases.
Some bound morphemes (inflections) are used, particularly plurals and the "ing" endings on verbs. Prepositions like "in" and "on" are also popular.