Learn Hardcore Turkish: O gün ne sinirli ne de üzgündü; oldukça sakindi. - That day he was neither angry nor sad; he was quite calm.

It corresponds to English neither ... nor ... and denies both items. Basic pattern: ne + X + ne (de) + Y, followed by a predicate in the positive form. Example with adjectives: O gün ne sinirli ne de üzgündü. With verbs: Ne ağladı ne güldü (neither cried nor laughed).

Why is there de after the second ne? Can I omit it?

The conventional form is ne ... ne de .... The de is optional: ne ... ne ... is also correct. Including de is very common and slightly smoother/formal. It appears as de (not da) here because of vowel harmony with ne (front vowel e).

Where does the meaning “was” come from in üzgündü/sakindi if there’s no separate verb?

Turkish uses a copular past suffix derived from idi attached to adjectives/nouns:

üzgün + idi → üzgündü (was sad)
sakin + idi → sakindi (was calm) This is how Turkish says “was X” without a separate verb.

Why is it -dü in üzgündü but -di in sakindi?

Because of vowel harmony: the last vowel of the stem determines the vowel of the copular past:

üzgün (last vowel ü) → -dü
sakin (last vowel i) → -di Also note voicing: after a voiceless consonant, it would surface as -tı/-ti/-tu/-tü (e.g., gençti).

Should I also add the past marker to the first adjective, as in ne sinirliydi ne de üzgündü?

You can. Ne sinirliydi ne de üzgündü is fully explicit; ne sinirli ne de üzgündü is also standard and natural due to parallelism/ellipsis. Adding -ydi to the first item can slightly emphasize the parallel past states.

Is oldukça the same strength as çok? What nuance does it have?

oldukça ≈ “quite/pretty/fairly/considerably,” typically weaker than çok (“very/very much”). Rough guide:

weaker → stronger: biraz < epey/oldukça ≲ bayağı (colloq.) < gayet/pek < çok Register-wise, oldukça feels neutral to slightly formal.

Where should oldukça go? Can I put it after the adjective?

Place it before the adjective: oldukça sakin. Don’t put it after: sakin oldukça means “as/so long as (someone) becomes calm,” because -dikçe/-dıkça is a separate temporal/concessive suffix on verbs (here from olmak), not the degree adverb.

Why is there a semicolon? Could I use a period or a comma instead?

The semicolon links two closely related independent clauses: the first sets up a contrast, the second gives the outcome. You could also write a period: O gün ne sinirli ne de üzgündü. Oldukça sakindi. A comma would be less standard since both sides are full clauses.

What exactly does O gün do? How is it different from bugün?

O gün = “that day,” pointing to a specific day (usually past or contextually defined). bugün = “today.” Here it frames the time for the states that follow.

Is the subject pronoun o missing? How do we know it’s “he/she/it”?

Turkish drops subject pronouns when context makes them clear. The predicate …-di signals third person past with an adjective/noun. Context tells you whether o refers to he/she/it.

Can I rewrite this with değil instead of ne ... ne de ...?

Yes, with two negatives: O gün sinirli değildi, üzgün de değildi; oldukça sakindi. Meaning stays the same; the original is more compact and idiomatic.

Can I use ne ... ne de ... with nouns and verbs too?

Yes.

Nouns: Ne doktor ne de hemşireydi (neither a doctor nor a nurse).
Verbs: Ne aradı ne de mesaj attı (neither called nor texted).

Does ne ... ne de ... require singular or plural agreement?

With third-person plural subjects, both singular and plural predicates occur; choice depends on whether you view the items as a single set or as separate actors. With adjectives and a singular subject (as here), you naturally use singular: üzgündü/sakindi.

What’s the nuance difference among sinirli, kızgın, and öfkeli? And üzgün vs kederli/üzüntülü?

sinirli: irritable, bad-tempered (can be a mood or a general disposition); context here = temporary mood.
kızgın: angry (often at someone/something specific).
öfkeli: furious/angry (stronger, more intense).
üzgün: sad, upset (common, neutral).
kederli/üzüntülü: sorrowful, mournful (more formal/literary).

Is there any ambiguity with o in O gün ... sakindi, since o can mean “that” or “he/she/it”?

No in practice. In O gün, o is a demonstrative modifying gün (“that day”). The subject pronoun o is simply omitted (understood from context). Native readers won’t confuse them.

Could I say the same idea with verbs like olmak or kalmak?

Yes, slightly more eventive:

O gün ne sinirli oldu ne de üzgün oldu; oldukça sakin kaldı. This emphasizes the (non-)occurrence of becoming those states, rather than describing them as static qualities.