Learn Hardcore German: Wir stornieren die Reservierung online, falls wir morgen doch nicht kommen. - We cancel the reservation online in case we don’t come tomorrow after all.

wenn is the default, very common word for if/when.
falls is a bit more formal and emphasizes a condition that may or may not happen (often like in case / if it turns out that).

So falls wir morgen doch nicht kommen = in case we don’t come after all tomorrow.
You could also say wenn wir morgen doch nicht kommen and it would still be correct and natural.

Why is the verb stornieren in position 2 in the first part, but at the end in the falls part?

German word order:

Main clause: the conjugated verb is in position 2 → Wir stornieren …
Subordinate clause (introduced by falls, wenn, weil, etc.): the conjugated verb goes to the end → … falls wir … nicht kommen.

That’s why you get kommen at the end of the falls clause.

Why is it die Reservierung (accusative) and not der Reservierung (dative)?

Because stornieren takes a direct object (what you cancel), which is in the accusative:

die Reservierung (accusative = same form as nominative for feminine nouns)

Dative would be used for an indirect object (to/for whom something is done), which isn’t the structure here.

What does doch add in doch nicht kommen?

doch is a common particle that often signals a contrast or change of expectation. Here it means something like:

after all
in the end
contrary to what we thought/said/planned

So morgen doch nicht kommen implies you currently expect to come, but there’s a possibility you won’t come after all.

Why is nicht placed before kommen?

In German, nicht usually comes:

before the part it negates, and
in subordinate clauses it often appears shortly before the final verb if the verb/action is being negated.

Here, what’s being negated is the action kommen, so you get nicht kommen = not come.

Is morgen flexible in position? Could it be earlier or later?

Yes, morgen can move, depending on what you want to emphasize. All of these are possible:

… falls wir morgen doch nicht kommen. (neutral)
… falls wir doch morgen nicht kommen. (slightly different emphasis)
Morgen stornieren wir die Reservierung online, … (emphasizes “tomorrow” in the main clause)

German is flexible, but the verb position rules (V2 in main clauses, verb-final in subordinate clauses) still apply.

Why is there a comma before falls?

German uses a comma to separate:

a main clause from a subordinate clause.

So you write: Wir stornieren …, falls …
This comma is required in standard German.

Can the sentence start with the falls clause?

Yes. Then the subordinate clause comes first, and the main clause still needs verb position 2—meaning the verb comes right after the first element (the whole falls clause counts as element 1):

Falls wir morgen doch nicht kommen, stornieren wir die Reservierung online.

The comma remains.

Is online treated like a normal adverb? Where does it go?

Yes, online functions like an adverb here (“in an online way / via the internet”). Common placements are:

Wir stornieren die Reservierung online. (very natural)
Wir stornieren online die Reservierung. (possible, slightly more emphasis on online)

In general, it often goes near the verb or near what it modifies, and native usage strongly favors … Reservierung online in this sentence.

Could you also say unsere Reservierung instead of die Reservierung?

Yes. die Reservierung is fine when it’s clear which reservation you mean (context does the work).
If you want to be explicit, you can say:

Wir stornieren unsere Reservierung online, falls …

Both are natural; unsere just adds clarity/emphasis.