"Kesa gelte sȳz issa."
Translation:This helmet is good.
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"Kesy" stands on it's own. As the -y ending gives away, it's its own noun. In this case, "this" isn't used as its own independent element of the sentence, but as a modifier for "gelte" - it's "this helmet", not "this (one)". And in that case, you use "kesa", which is declined like an adjective.
It isn't taught as a specific skill (although it is listed in the wiki). I think it's just assumed that you'll pick up on the fact that -y is only ever a noun ending, while -a is also a common adjective ending. It's also something that is a logical consequence of having a language that is declined to this degree. My own native language has different words for "this (subject of a sentence)" and "this (modifier of a noun)" as well, and it's also the endings that make the difference.
Assuming is never a good approach if you want to teach something. Besides, even if it was logical that "this" as a subject of the sentence and "this" as a modifier of a noun have different words, it is not very useful knowledge if one is not taught what exactly the form of the latter one is.