Syntax was a composition of rules for sentences in a language.
By 2151, Klingonese had eighty polyguttural dialects constructed on an adaptive syntax. Upon encountering Klaang that year, the universal translator of Hoshi Sato wouldn't lock on to his dialect, so the syntax didn't align. (ENT: "Broken Bow")
In 2370, the Skrreean language had such a different syntax and grammar compared to languages in the Federation database, that it took the universal translators aboard Deep Space 9 longer than usual to translate their language. (DS9: "Sanctuary")