Morphosyntax is another word for grammar.
Grammar can be divided into morphology and syntax. Morphology is the study of words and their rules of formation. And syntax is the study of sentences and their rules of formation. Essentially, morphology and syntax are studies of the same thing – formation rules of a language – but at differing “levels”.
By calling it by the transparent term morphosyntax we are highlighting this dualism.
When we talk about word-formation (morphology) we use terms like
And when we talk about sentence-formation (syntax) we use terms like
The term verb unfortunately has “double duty” for word-forming and sentence-forming. So when using the term be careful and clear to your reader/listener as to which meaning of the verb you are trying to convey.
Note also that the sentence-formation terms do not appear in dictionary definitions, indicating most clearly the idea that dictionaries are about words, and not sentences.