Learn Hardcore German: Im Video sehen wir später die Wiederholung der Szene, und viele merken, dass die Entscheidung nicht unfair war. - In the video we later see the replay of the scene, and many realize that the decision was not unfair.

Grammatically, Video is neuter (das Video). After the preposition in with a static location (in meaning in/inside without movement), you use the dative case: dem Video.
So im Video literally means in the video.

Why does the sentence start with Im Video instead of the subject wir?

German often puts something other than the subject in the first position for emphasis or to set the scene: time, place, etc.

Im Video sehen wir später die Wiederholung … (place first: In the video, we later see the replay…)
Später sehen wir im Video die Wiederholung … (time first: Later, we see in the video…)
Wir sehen später im Video die Wiederholung … (subject first: more neutral order)

All three are grammatically correct.
Starting with Im Video emphasizes that this is something we see in the video (as opposed to live, for example).

Why is it sehen wir and not wir sehen right after Im Video?

German main clauses follow the verb-second rule (V2):

Exactly one “big chunk” (word or phrase) comes first.
Then the conjugated verb.
Then the rest (subject, objects, adverbials, etc.).

In the sentence:

First chunk: Im Video
Second: sehen (the finite verb)
Then: wir (the subject) and the rest

So:

Im Video sehen wir später die Wiederholung … (correct V2 order)
If you started with the subject, it would be Wir sehen im Video … – that still respects the verb-second rule, because Wir is then in position 1 and sehen is in position 2.

Where exactly does später have to go, and could I move it?

Später is an adverb of time. In German, time, manner, and place adverbs are fairly flexible in position. In this sentence, common options are:

Im Video sehen wir später die Wiederholung der Szene … (as given)
Im Video sehen wir die Wiederholung der Szene später …
Später sehen wir im Video die Wiederholung der Szene …
Wir sehen später im Video die Wiederholung der Szene …

All are grammatical. Differences are about nuance/emphasis, not correctness.
The two most natural for neutral speech here are probably:

Im Video sehen wir später die Wiederholung der Szene …
Später sehen wir im Video die Wiederholung der Szene …

What case is die Wiederholung der Szene, and why is it der Szene?

die Wiederholung der Szene contains:

die Wiederholung – nominative/accusative singular feminine (die is the direct object of sehen)
der Szene – feminine genitive singular after Wiederholung

Structure: die Wiederholung + Genitive = the repetition/replay of X

die Wiederholung der Szene = the replay of the scene

In English we use of; in German this can become a genitive phrase without a preposition:

die Wiederholung der Szene (more formal/standard)
die Wiederholung von der Szene (possible, but more colloquial; many teachers prefer genitive here)

Because Szene is feminine (die Szene), its genitive singular is der Szene.

Why is there a comma before und? In English we often don’t use one before and.

In German, a comma is mandatory when und connects two main clauses.

Here we have:

Im Video sehen wir später die Wiederholung der Szene
viele merken, dass die Entscheidung nicht unfair war

Each part has its own conjugated verb (sehen and merken) and its own subject (wir and viele). So they are two independent main clauses joined by und, and the comma is required:

Im Video sehen wir später die Wiederholung der Szene, und viele merken, dass …

Who are viele, and why can you just say viele merken without a noun?

Viele here is a pronoun meaning many (people). The noun Leute (people) is understood and therefore omitted:

viele (Leute) merken, dass … = many (people) realize that …

German often drops the noun when it is obvious:

Viele sind schon da. – Many are already there.
Einige haben abgesagt. – Some have canceled.

So viele merken is natural and idiomatic.

Why is it merken, dass … and not something like realisieren or kapieren?

Several verbs could work, but they differ in tone:

merken, dass … – neutral, very common: to notice / realize that …
bemerken, dass … – a bit more “formal”, often for noticing something perceptible
realisieren, dass … – exists, but sounds somewhat more formal/foreign; less everyday than merken
kapieren, dass … – colloquial: to get / to catch on that …

In a neutral narrative sentence like this, merken, dass … is the most natural everyday choice.

Why is war at the very end of dass die Entscheidung nicht unfair war?

dass die Entscheidung nicht unfair war is a subordinate clause introduced by dass. In German subordinate clauses (introduced by dass, weil, wenn, obwohl, …), the conjugated verb goes to the end:

dass
- subject + other elements + verb (final position)

So:

dass – subordinator
die Entscheidung – subject
nicht unfair – predicate adjective with negation
war – conjugated verb, at the end

Word order pattern: …, dass S O Adv Prädikat V

Why say nicht unfair instead of simply fair?

Both are possible, but they differ slightly in meaning/nuance:

dass die Entscheidung fair war – that the decision was fair
→ directly states fairness; positive evaluation.
dass die Entscheidung nicht unfair war – that the decision was not unfair
→ weaker, more defensive: the decision might not feel completely fair, but at least it isn’t unfair.

Nicht unfair is a kind of litotes (negating the negative). It often suggests:

The situation may be borderline or complex.
People accept the decision as acceptable, not necessarily enthusiastically fair.

Why is the negation nicht placed before unfair and not somewhere else, like … unfair nicht war?

In German, nicht usually comes before the element it negates. Here we are negating the adjective unfair, so:

nicht unfair = not unfair

The pattern in a sein-sentence (with adjectives) is:

Die Entscheidung war unfair.
Die Entscheidung war nicht unfair.

In a subordinate clause with dass, the finite verb moves to the end, but the nicht stays before the predicate unfair:

dass die Entscheidung nicht unfair war

Forms like dass die Entscheidung unfair nicht war are ungrammatical.