The search for Baby Grammar
If you think English grammar is hard, get a load of Aborigine grammar.
One major benefit of grammar, as we described it, is in the redundancy it provides for decoding or going from surface structure to deep structure.
The search for the "Holy Grail" to the developmental psycholinguist was to find and define a "baby grammar." In one description there were identified two classes of words, ”Pivot and Open." Pivot words were those used frequently by the child and these were small in number. Open words included all the rest of the child's lexicon. The big restriction (grammar) was that Open words were never combined in the same sentence.
This of, course would have been an explicit example of grammar. Unfortunately, although this was described in one study, it was not reduplicated in others. Apparently, the Holy Grail is yet to be found.