Learn Finnish: Uusi versio ei ole yhtä kestävä kuin vanha. - The new version is not as durable as the old one.

It uses yhtä (equally) followed by the adjective kestävä (durable) and then kuin (as). Together, yhtä kestävä kuin creates the equative comparison 'as durable as'. This structure works exactly the same whether the sentence is positive or negative.

Isn't yhtä the partitive form of the number one (yksi)?

Yes, it is! In this comparison structure, you can think of it as historically meaning 'of one measure' or 'to the same degree'. Today, it functions as a fixed adverb meaning 'equally' or 'as' when paired with kuin.

Since the sentence is negative (ei ole), shouldn't the adjective kestävä be in the partitive case?

No, because the subject Uusi versio (The new version) is a singular, countable thing. When describing a singular countable noun, the adjective stays in the basic nominative form (kestävä), even in a negative sentence. You would only use the partitive if the subject were an uncountable mass noun, like water or time.

The English translation says 'the old one'. Why is there no word for 'one' after vanha?

Finnish doesn't use a placeholder word like 'one' in these situations. You simply leave the adjective vanha on its own, and the listener naturally understands from the context that you mean 'the old version'.