Learn Hardcore German: Ich storniere die Buchung online.

Buchung is a feminine noun, so its definite article is die. It’s the direct object of stornieren, so it’s in the accusative case. For feminine singular, nominative and accusative are both die, so you see no change.

Is stornieren a separable verb? What happens in subordinate clauses?

It’s not separable. In a subordinate clause, the conjugated verb goes to the end:

…, weil ich die Buchung online storniere. In the perfect, the participle storniert goes to the end:
Ich habe die Buchung online storniert.

Where can online go? Is the given order the only correct one?

No, German allows flexibility. Neutral is:

Ich storniere die Buchung online. Other options:
Ich storniere online die Buchung. (emphasis on doing it online; slightly marked)
Online storniere ich die Buchung. (fronted for emphasis) All are grammatical; the default places short adverbs like online near the end before final verbs/participles.

How do I negate this, especially if I mean “not online” versus “not canceling”?

Not canceling at all: Ich storniere die Buchung nicht.
Not online (but by phone/in person): Ich storniere die Buchung nicht online. Placing nicht right before online targets the manner.

Can I replace die Buchung with a pronoun?

Yes. Buchung is feminine, so accusative pronoun is sie:

Ich storniere sie online. Pronouns tend to appear earlier than longer adverbials.

How can I say this more politely to a hotel or airline?

Ich möchte die Buchung online stornieren.
Ich würde gern/meine Buchung online stornieren.
Formal request to staff: Könnten Sie meine Buchung online stornieren?

What’s the difference between Buchung and Reservierung?

Buchung: a booking/purchase that’s often confirmed and possibly paid (e.g., flight, hotel, order). More transactional.
Reservierung: a reservation/holding of a spot (table, seat, appointment), not necessarily paid. Often interchangeable for hotels/flights, but context matters.

How do stornieren, absagen, kündigen, widerrufen, annullieren, abbestellen differ?

stornieren: cancel a booking/order/transaction (hotel, flight, order).
absagen: call off an event/appointment: einen Termin absagen.
kündigen: terminate a contract/subscription/job: den Vertrag kündigen.
widerrufen: revoke within a legal cooling-off period (consumer rights).
annullieren: officially void/cancel (flights/trains, formal).
abbestellen: cancel an order/subscription delivery (magazine, food).

How do I say it in the past or the future?

Spoken past (Perfekt): Ich habe die Buchung online storniert.
Narrative past (Präteritum): Ich stornierte die Buchung online. (rarer in speech)
Future: Ich werde die Buchung online stornieren. Note: Present can express near future with a time word: Ich storniere die Buchung morgen online.

Should online be capitalized?

As an adverb, it’s lowercase: online. At the start of a sentence, it’s capitalized by position: Online storniere ich… In compounds as a modifier before a noun, you’ll see Online-Buchung or Onlinebuchung; both are common.

Do I need an article here? Could I say “Ich storniere Buchung”?

You need an article with countable nouns. Use:

definite: die Buchung (a specific one),
indefinite: eine Buchung (some booking),
possessive: meine Buchung. Bare singular without an article is generally ungrammatical in this context.

What happens if I add time or other adverbs? Any word-order rule to follow?

A common guideline is Time–Manner–Place. Online is manner. So:

Ich storniere die Buchung morgen online. If you also add a place: Ich storniere die Buchung morgen online zu Hause. (Time–Manner–Place) This is a tendency, not a hard law; emphasis can change order.

How do I form the passive voice?

Present passive: Die Buchung wird online storniert.
Perfect passive: Die Buchung ist online storniert worden. This focuses on the booking rather than who cancels it.

Any quick pronunciation tips for the tricky words?

Ich: soft “ch” like the hissing in “hue” (not a hard k).
storniere: stress on -nie-: shtor-NEE-reh.
Buchung: long u (boo), ch like in Scottish “loch,” final -ung like “oong.”
online: typically ON-line (German o as in “off”).

Is stornieren formal or neutral? Are there colloquial alternatives?

Stornieren is neutral and standard in customer-service contexts. Colloquial anglicisms like canceln exist but are informal and best avoided in careful usage. There’s no reflexive form here; you don’t say ich storniere mich.