После перехода подачи друг ошибся.

Because после requires the genitive case.

The dictionary form is переход, but after после it becomes перехода.

A few similar examples:

• после урока = after the lesson
• после игры = after the game
• после перехода = after the transition/change

So this is a very standard pattern: после + genitive.

Here, подачи depends on перехода, not directly on после.

You can think of the structure like this:

• после [перехода подачи]

In other words:

• переход чего? — подачи

Russian often links nouns this way with the genitive, where English might use of or a noun modifier.

So grammatically, this is a kind of genitive chain:

• after [the transition/change of the serve/submission/etc.]

Because друг is in the nominative case, and it matches the verb as the person doing the action.

The basic sentence skeleton is:

• друг ошибся = the friend made a mistake

Everything before that, После перехода подачи, is just setting the time/context: after ...

So the main clause is:

• друг ошибся

Russian has no articles.

So друг can mean:

• a friend
• the friend
• sometimes just friend

Which one is meant depends on context.

English speakers often want a one-to-one match with a/the, but Russian simply does not mark that in the same way.

On its own, друг usually just means friend.

If you want to be fully explicit, Russian often says:

• мой друг = my friend

However, in context, Russian can sometimes omit мой if it is already obvious whose friend is meant.

So:

• мой друг = explicit
• друг = just friend, or a context-dependent the/my friend

No. In this verb, -ся is just part of the normal dictionary form:

• ошибиться = to make a mistake / to be mistaken

It is best learned as a whole verb, not as a literal verb + himself construction.

This is very common in Russian: some verbs are reflexive in form but are translated naturally in English without self.

So:

• друг ошибся = the friend made a mistake

not something like the friend mistaken himself.

Ошибся is:

• past tense
• masculine singular
• perfective aspect

The perfective here shows one completed mistake.

Compare:

• ошибся = made a mistake, got it wrong once
• ошибался = was making mistakes / used to make mistakes / kept being wrong

So in this sentence, Russian presents the mistake as a single completed event.

Because in Russian, singular past-tense verbs agree in gender.

Here:

• ошибся = masculine
• ошиблась = feminine
• ошиблось = neuter
• ошиблись = plural

So ошибся tells you the subject is grammatically masculine.

That is why it fits well with друг.

Yes. Russian word order is flexible.

This sentence begins with:

• После перехода подачи

That puts the time/context first: after the transition/change of the serve/submission/etc.

A different order is also possible:

• Друг ошибся после перехода подачи.

Both are grammatical, but the emphasis changes a little.

• После перехода подачи друг ошибся = first set the scene, then say what happened
• Друг ошибся после перехода подачи = first mention the subject, then add when it happened

Normally, no.

A fronted prepositional phrase like this usually does not need a comma in neutral writing:

• После перехода подачи друг ошибся.

You might see a comma only if the writer wants a special pause or emphasis, but that is not the default.

Grammatically, yes.

But переход подачи sounds somewhat context-specific or technical. It feels more natural if the surrounding context is clear, for example in sports or another specialized setting.

So a native speaker would probably understand the grammar immediately, but might need the broader context to know exactly what kind of подача and what kind of переход are meant.