"Cream is tasty, water is not tasty!"
Translation:Tha uachdar blasta, chan eil uisge blasta!
The reason that it's not needed in the second part:"chan eil uisge blasta" - is that the "is not" part is contained in "Chan eil".
It is needed in the first part, because otherwise you would have "uachdar blasta" - which means "tasty cream". To say "cream IS tasty", we have "THA uschdar blasta"
In some cases, when we're learning a new language there is a direct word-for-word equivalence, but sometimes there isn't.
Chan eil uisge blasta means (approximately) "Is not / water/ tasty". Now in English, putting the verb at the start would make it read like a question "Is not water tasty?", in Gaelic, it's not a quesion - it's a normal statement or opinion - Water is not tasty. So when it's a negative statement (is not), Gaelic uses "chan eil" .
I'll qualify my answer above by saying that there's more to it than that in terms of negating things, (lots) but I don't want to give a more complex and possibly offputting answer, because I'm still learning myself