supootu no siai de ha, aite no ugoki wo yoku mimasu.

This is a very common question.

• で marks the place/situation where an action happens: in/during a match
• は marks that whole phrase as the topic or adds contrast/emphasis

So:

• 試合で = in a match / during a match
• 試合では = as for in a match / in a match, ...

Here, では makes during a sports match the setting the speaker wants to talk about. It can also sound slightly contrastive, like when it comes to matches.

The particle の connects nouns. It often works like:

• of
• 's
• a noun used like an adjective

So スポーツの試合 means something like:

• a sports match
• literally, a match of sports

It tells you what kind of match it is.

Here の shows possession or association.

• 相手 = opponent
• 動き = movement(s)
• 相手の動き = the opponent's movements

So this の is very similar to English 's.

Because 動き is the direct object of 見ます.

• 何を見ますか。 = What do you look at/watch?
• 相手の動きを見ます。 = I look at the opponent's movements.

So を marks the thing being watched or observed.

よく is an adverb, and adverbs usually come before the verb they modify.

• よく見ます = look carefully / watch closely / often watch, depending on context

In this sentence, because of the meaning, よく most naturally means carefully or closely, not just often.

So the idea is:

• I watch the opponent's movements carefully.

It can mean either in different sentences, which is why learners often notice it.

Common meanings of よく include:

• often
• well
• carefully / closely

In this sentence, 相手の動きをよく見ます, the natural meaning is:

• watch closely
• observe carefully

That is because in a sports match, you pay close attention to the opponent's movements. Often watch is possible grammatically, but it is less natural here.

見ます is the non-past polite form. It can express:

• a habitual action
• a general truth
• future action, depending on context

In this sentence, it sounds like a habitual/general statement:

• In sports matches, I watch the opponent's movements carefully.

If you said 見ています, it would sound more like:

• I am watching
• or I regularly do this, with a stronger ongoing feel depending on context

Both can be possible in some situations, but 見ます is very natural for a general statement.

Here it is closer to:

• watch
• look at
• observe

Japanese 見る / 見ます covers a wider range than any one English verb.

In this sentence, the meaning is not just accidentally seeing something. It is intentional, so watch or observe is best.

Japanese often leaves out the subject when it is understood from context.

So this sentence could mean:

• I watch the opponent's movements carefully in a sports match.
• You watch the opponent's movements carefully in a sports match.
• We watch...
• or even a general one watches...

In practice, if this is a textbook sentence, it often gets translated with I because English usually needs a subject.

相手 has a broad meaning. It can mean:

• opponent
• the other person
• partner
• the person you are dealing with

In a sports context, 相手 naturally means opponent.

So in this sentence, 相手の動き means the opponent's movements.

Japanese word order is more flexible than English, as long as the particles stay correct.

This sentence could be rearranged in some ways, for example:

• スポーツの試合では、相手の動きをよく見ます。
• 相手の動きを、スポーツの試合ではよく見ます。

These are grammatically possible, but the original order is very natural.

The verb usually comes at the end, and the particles show the role of each part.

The comma is mainly for readability and rhythm.

• スポーツの試合では、相手の動きをよく見ます。

It helps separate the topic/setting from the main statement. Japanese commas are often more flexible than English commas, so this comma is helpful but not absolutely required in every context.

Yes, it is natural and easy to understand.

• スポーツ = sports
• 試合 = match, game, contest

So スポーツの試合 simply means a sports match/game.

Depending on context, native speakers might also just say 試合 if it is already obvious they are talking about sports. For example:

• 試合では、相手の動きをよく見ます。

That would often be enough.