Navigate to Noun Declension Engine from grammar articles

In this update I've improved navigation and organization of exercises.

You can now easily navigate to the Noun Declension Engine directly from the grammar articles. This makes it much more convenient to practice noun declension while reading about grammar rules. You can see it in the video below.

You can still choose manually what noun declension exercises you want to practice.

I reorganized the grouping of exercises. Now noun exercises and possessive pronoun exercises are separated.