It's indeed very tricky. The word هٰذِهِ = this (fem.) is a demonstrative. If it is followed by a definite noun (like الْموزة = the banana), it is considered as a determiner. Therefore هٰذِهِ الْموزة which is literally "this the banana" means "this banana". If هٰذِهِ is not followed by a definite noun, it is considered as a pronoun. For instance هٰذِهِ موزة which is literally "this a banana" means "this is a banana". In other words:

• هٰذِهِ الْموزة غالْية = "This banana is expensive"
• هٰذِهِ موزة غالْية = "This is an expensive banana"