"This tea is pretty hot."
Translation:Tämä tee on melko kuumaa.
Because liquids are uncountable nouns (as are rice, salt, gas, flour, etc.) and even if you can order at a restaurant "yksi tee, kiitos", it is actually abbreviation of "one cup of tea" and after it is served and you take a sip of it, you cannot say "tämä tee on melko kuuma." You must say "tämä tee on melko kuumaa." Even after you drink all of it while it's hot, you still say "tee oli (was) kuumaa."
oh thanks. uncountable nouns, numbers, quantities and actions which may or may not have completed ... vaikea mutta kiehtova. It does not help at all to take these into English and look for equivalents there.
Sorry I don't really know the answer but i checked here and found nothing relevant https://uusikielemme.fi/finnish-grammar/grammatical-cases/the-partitive-case-partitiivi
But I found a site which had examples "Vesi on kylmää" and "Viini on kalista" which look the same, called these sentences 'Millaista / what kind of' situations , 'predicative complement', that's my working theory until an expert comments!