Learn Hardcore Turkish: Kestane sıcakken daha lezzetli oluyor. - Chestnuts are more delicious while they are hot.

With adjectives/nouns: sıcak + (i)ken → sıcakken.
With verbs, you typically attach -ken to the aorist 3rd-singular form: gelir + ken → gelirken (while coming), okur + ken → okurken (while reading). You’ll also hear the progressive variant with -yorken in speech, e.g., geliyorken.

Why is it written as sıcakken with a double k?

Because it comes from sıcak + iken. In everyday Turkish, iken contracts to -ken, and when the word already ends in k, you get kk: sıcakken. The same happens in words like soğukken (from soğuk + iken) and yokken (from yok + iken).

Can I say sıcak iken, sıcak olduğunda, or sıcak olunca instead of sıcakken?

Yes, all are correct with slight nuances:

sıcak iken: more formal/literary.
sıcakken: the common, natural everyday form.
sıcak olduğunda: when it is hot (neutral, a bit heavier).
sıcak olunca: once/when it becomes hot, with a mild sense of a change/threshold.

Why is kestane singular? Shouldn’t it be kestaneler?

Turkish often uses the singular to talk about a food or thing in general (generic sense). Kestane here means chestnut(s) as a kind.

Kestane sıcakken… = generic, kind-level.
Kestaneler sıcakken… is also fine and focuses a bit more on individual chestnuts. Both are natural.

Should the verb be plural (oluyorlar) because chestnuts are plural?

No. With non-human plural subjects, Turkish normally uses the 3rd person singular verb:

Kestaneler… daha lezzetli oluyor. Using oluyorlar is possible but less common with non-humans unless you want to emphasize individual items. With human subjects, plural agreement is the norm.

Why is there no the or a before kestane?

Turkish has no definite article. Indefinite bir exists, but bir kestane would mean a chestnut (one chestnut), which is not the intended generic meaning here. So bare kestane covers the idea naturally.

Why use oluyor instead of olur or lezzetlidir?

All three are possible; they differ in tone:

… daha lezzetli olur: aorist (general truth, neutral, a bit more standard).
… daha lezzetli oluyor: progressive used conversationally to state habitual tendencies or generalizations; very common in speech.
… daha lezzetlidir: with -dir, sounds formal/definitive or emphatic. The sentence with oluyor reads as a natural, experience-based generalization.

Can I drop the verb and just say Kestane sıcakken daha lezzetli?

Yes. Predicate adjectives can stand without an explicit copula in the present. Kestane sıcakken daha lezzetli is fine, especially in speech or casual writing. Including olur/oluyor simply makes the sentence a bit fuller and is also very natural.

What does daha do? How do I make comparatives and superlatives?

daha before an adjective makes a comparative: daha lezzetli = tastier/more delicious.
en makes the superlative: en lezzetli = the most delicious. To say than X, use -den/-dan: Kestane, fındıktan daha lezzetli (Chestnuts are tastier than hazelnuts).

Can I move words around? Where does daha go?

daha always precedes the adjective: daha lezzetli.
Adverbials like sıcakken can move: Sıcakken kestane daha lezzetli oluyor or Kestane, sıcakken, daha lezzetli oluyor.
The verb typically stays at the end.

How do I pronounce the tricky letters in sıcakken and lezzetli?

ı (dotless i): a back, unrounded vowel; think a relaxed, short “uh,” but farther back.
c: like English j in jam.
kk: a longer, geminated k; keep it tight with no vowel between.
zz in lezzetli is a clear voiced z; say both z’s distinctly.

Could I use warm instead of hot?

Yes:

ılık = warm, so ılıkken = when warm. You can also intensify hot: sıcacık (piping hot), or use the adverbial duplication sıcak sıcak:
Kestane ılıkken de güzel olur, ama sıcacıkken en güzeli.
Kestane sıcak sıcak daha lezzetli olur.

How do I say the opposite, like when cold?

Use soğuk:

soğukken = when cold. For example: Kestane soğukken o kadar lezzetli olmuyor (It’s not as tasty when cold).

Can -ken attach to nouns too?

Yes. It often means when one was/while being X:

çocukken = when (I/you/he…) was a child.
yoldayken = while on the road. So sıcakken fits the same pattern but with an adjective.

Could I express the idea with yenince or a different structure?

Yes. You can focus on the action of eating:

Kestane sıcak yenince daha lezzetli oluyor (When eaten hot, chestnut is tastier). Another very natural paraphrase is to talk about taste:
Kestanenin tadı sıcakken daha güzel olur (The taste of chestnut is better when hot).

Does oluyor mean becomes here, or just is?

Literally it’s becomes, but in everyday Turkish … daha lezzetli oluyor often functions like is/tastes in general statements about tendencies. It implies the result you typically get in that situation, not necessarily a dramatic change right now.