The two-word stage is the period in a child's life, usually around 18-24 months old, when they start to use two-word sentences to communicate.
Morpheme: The smallest grammatical unit in a language that carries meaning. For example, 'un-' in 'unhappy'.
The arrangement of words and phrases to create well-formed sentences in a language.
This occurs when grammar rules are incorrectly generalized to irregular cases where they do not apply. For instance, saying "runned" instead of "ran".