Learn Hardcore Japanese: supootu no siai de ha, aite wo sonkeisurukoto ga taisetu desu. - In sports matches, it is important to respect your opponent.

スポーツの試合 literally = “match of sports” → “a sports match / a sporting event”.
This is the normal way to say it. You generally can’t just stick two nouns together in Japanese the way English does (like sports match).
スポーツ試合 sounds wrong/unnatural; Japanese usually needs の (or another linker) between most nouns.

So N1 の N2 is often “N2 related to N1” or “N2 of N1” → スポーツの試合 = a match in the context of sports.

Why is it では (では) in スポーツの試合では instead of just で or just は?

では is actually で + は.

で marks the place or situation:
スポーツの試合で = “in a sports match / in the context of a sports game”.
は marks the topic or contrast:
スポーツの試合では = “As for in sports matches,” or “In sports matches (in particular), …”

So では here:

Sets the context: in the situation of sports matches.
Slightly emphasizes/contrasts that context: “In sports matches (at least / especially), it’s important…”

If you said only スポーツの試合で, it would be grammatically OK, but without は you lose that “as for / in that setting” topic feel.

What exactly does で mean here? Is it “at” or “in”?

Here で marks the place/situation where something happens.

スポーツの試合で can be understood as:
- “at sports matches”
- “during sports matches”
- “in the context of sports competitions”

In English you might translate it as “in”, “at”, or “during”, but in Japanese で is a single particle that covers all those nuances: “in the setting of X”.

What does 相手 mean exactly? How is it different from 敵?

相手 (あいて) is a very flexible word meaning things like:

opponent
partner
the other person (in a conversation, game, relationship, etc.)

In this sentence, 相手 = “your opponent” in a match.

敵 (てき), on the other hand, means:

enemy
foe (stronger, more hostile nuance)

So:

In sports, saying 相手 is normal and neutral/polite:
試合の相手 = the opponent in a match.
敵 sounds like a real “enemy”, more like a hostile or adversarial relationship.

The sentence uses 相手 because it’s talking about respect; you usually respect an 相手, not an 敵.

Why is it 相手を尊敬する and not 相手に尊敬する?

尊敬する (to respect) is a transitive verb that takes a direct object with を.

So the pattern is:

A を尊敬する = respect A

Examples:

先生を尊敬しています。
I respect my teacher.
相手を尊敬することが大切です。
Respecting your opponent is important.

に is not used here because 尊敬する doesn’t take an indirect object the way some English verbs do. The person you respect is marked by を.

What does 尊敬する literally mean, and how is it different from 尊重する?

Both involve “respect”, but they focus on slightly different things.

尊敬する (そんけいする)
→ to respect, admire, look up to someone
Nuance: you think highly of the person, their character, abilities, or status.
尊重する (そんちょうする)
→ to respect, to value (a decision, opinion, rights, rules)
Nuance: you honor / give weight to something (a rule, opinion, will, privacy).

Examples:

両親を尊敬しています。
I respect my parents. (admiration)
相手の意見を尊重します。
I respect the other person’s opinion. (I take their opinion seriously)

In your sentence, it’s about having respect toward the opponent as a person, so 尊敬する is the natural choice.

Why do we need こと after 尊敬する? Why not just 相手を尊敬が大切です?

You can’t attach が大切です directly to a verb phrase. You need to turn the verb phrase into a noun-like thing (a “nominalized” phrase).

相手を尊敬する = “to respect your opponent” (verb phrase)
相手を尊敬すること = “the act of respecting your opponent” (now it behaves like a noun)

Then you can say:

相手を尊敬することが大切です。
“The act of respecting your opponent is important.” → “It’s important to respect your opponent.”

相手を尊敬が大切です is ungrammatical because 尊敬 by itself is a noun “respect” without any verb linking it to 相手. You need 尊敬する to show “respecting (someone)”.

What’s the difference between using こと and の in 相手を尊敬すること / のが大切です?

Both こと and の can nominalize verbs (turn “to do X” into a noun phrase).

相手を尊敬することが大切です。
相手を尊敬するのが大切です。

In this kind of general, somewhat formal statement, こと is more standard and neutral, and is often preferred in writing or more formal speech.

Nuance difference (very roughly):

こと: a bit more abstract / formal / “concept-like”.
→ “The act / the idea of respecting your opponent is important.”
の: can feel slightly more concrete / casual / personal in many contexts.

In textbooks and polite explanations, you will most often see ～することが大切です. Both are grammatically correct, though.

What is 大切 grammatically? Is it an adjective? How is it different from 重要 or 大事?

大切 (たいせつ) is a na-adjective (形容動詞).

Dictionary form: 大切だ
Polite form: 大切です
Modification: 大切な人, 大切なこと

Meaning: important, precious, valuable, something you should take care of.

Comparison:

大切:
“important / precious / dear” – often with an emotional or caring nuance.
- 家族はとても大切です。
  Family is very important (and dear to me).
重要 (じゅうよう):
“important / significant / of great consequence” – sounds more formal/academic.
- これは重要な問題です。
  This is an important issue.
大事 (だいじ):
Very close to 大切; “important, serious, precious”. Often used in daily speech.
- 健康が大事です。
  Health is important.

In 大切です, the tone is: “This is something you should really value / take seriously.”

Could I say 相手を尊敬することは大切です instead? What’s the difference between が and は here?

Yes, you can say:

相手を尊敬することは大切です。

Both are correct, but the nuance changes slightly.

相手を尊敬することが大切です。
- が tends to mark what is being presented as important / new information.
- “It is respecting your opponent that is important.”
相手を尊敬することは大切です。
- は marks the topic; it can also sound more general or contrastive.
- “As for respecting your opponent, it is important (at least / generally).”

In many everyday situations, they’re almost interchangeable, but:

が: emphasizes that this thing (respecting your opponent) is what’s important.
は: can sound a bit more like a general rule or a contrast with other things.

Why is there a comma after では: スポーツの試合では、相手を…? Is it necessary?

The comma (、) in Japanese is mostly about readability and rhythm, not strict grammar.

スポーツの試合では、相手を尊敬することが大切です。
スポーツの試合では相手を尊敬することが大切です。

Both are correct. The comma just visually separates the topic phrase (スポーツの試合では) from the main statement.

Writers often put a comma:

after longer topic phrases
after ～では, ～には, ～のときは, etc.

It’s not “required” by grammar, but it’s very natural and common.

Can I say 大切だ instead of 大切です? And can I drop だ/です altogether?

大切です: polite/formal style (です・ます form).
大切だ: plain style (used in casual speech, or in written explanatory texts, etc.).

So:

Polite: スポーツの試合では、相手を尊敬することが大切です。
Plain: スポーツの試合では、相手を尊敬することが大切だ。

Dropping だ/です completely (大切。) is usually not done in a normal, full sentence like this. You may see predicate adjectives/nouns without だ in:

headlines
very informal note-like writing
some stylistic literary uses

But as a normal sentence, you should keep だ or です. Here, 大切です is the standard polite form.