هٰذَا لِجَامٌ — This is a bridle.
هٰذَا: demonstrative, indeclinable, in the nominative position as mubtadaʾ.
لِجَامٌ: predicate (khabar), nominative with visible ḍamma and tanwīn.
The tanwīn reflects its ordinary fully declinable usage. This sentence is a teaching example, not a quotation from the book.