premise: An island grammar is a grammar that only describes a small chunk of the underlying language. It is used in language parsing in situations where there is no requirement for checking the entire syntax of a provided text. Island grammars can be extended with the use of a bridge grammar.
hypothesis: The main topic of this article is sports.
reason: It's definitely incorrect because the main subject is language/grammar. It's difficult because verifying the statement requires a high level understanding of the entire text, and it might not know what "main topic" means exactly.