Przeciwnik udaje, że nie oszukuje.

Because że means that and introduces a subordinate clause.

So the structure is:

• Przeciwnik udaje = The opponent pretends
• że nie oszukuje = that he/she is not cheating

In Polish, udawać, że... is a very common pattern meaning to pretend that...

Examples:

• Udaje, że śpi. = He/She pretends to be sleeping.
• Udają, że nie wiedzą. = They pretend that they do not know.

Because after że, Polish normally uses a full conjugated verb, not the infinitive.

So:

• udaje, że nie oszukuje = pretends that he/she is not cheating

Not:

• udaje, że nie oszukiwać ❌

Compare:

• Chce oszukiwać. = He/She wants to cheat.
  Here the infinitive works because chce often takes an infinitive.

• Udaje, że nie oszukuje. = He/She pretends that he/she is not cheating.
  Here udawać takes a clause with że.

Because Polish often drops subject pronouns when they are clear from the verb form or context.

In oszukuje, the ending already tells you it is he/she/it in the present tense. Since the subject is already understood to be przeciwnik, Polish does not need to repeat on.

So Polish prefers:

• Przeciwnik udaje, że nie oszukuje.

Instead of the more explicit:

• Przeciwnik udaje, że on nie oszukuje.

Adding on is possible, but it usually adds emphasis or contrast, for example:

• Przeciwnik udaje, że on nie oszukuje, tylko inni.
  = The opponent pretends that he is not cheating, only others are.

It is nominative singular, because it is the subject of the sentence.

Basic dictionary form:

• przeciwnik = opponent, adversary

In this sentence:

• Przeciwnik = the opponent
• udaje = pretends

So przeciwnik is the one performing the action.

Udaje is the 3rd person singular present tense of udawać.

Here, udawać means:

• to pretend
• to act as if

So:

• udaje, że... = he/she pretends that...

Be careful: udawać can also appear in other meanings in different contexts, but with że it very often means pretend that.

Examples:

• Udaje chorego. = He is pretending to be ill.
• Udaje, że nic nie widzi. = He pretends that he sees nothing.

In Polish, nie normally goes directly before the verb it negates.

So:

• oszukuje = is cheating / cheats
• nie oszukuje = is not cheating / does not cheat

That is the standard way to negate a verb in Polish.

Examples:

• Czyta. = He/She is reading.

• Nie czyta. = He/She is not reading.

• Wie. = He/She knows.
• Nie wie. = He/She does not know.

It can mean either, depending on context.

Polish present-tense imperfective verbs often cover both:

• an action happening now
• a habitual or general action

So nie oszukuje can mean:

• is not cheating
• does not cheat

In your sentence, English would often choose is not cheating because the idea is about what the opponent claims or pretends at that moment, but context decides.

Because Polish usually uses the imperfective verb in the present tense for ongoing, repeated, or general actions.

The verb pair is roughly:

• oszukiwać = imperfective
• oszukać = perfective

The form oszukuje comes from oszukiwać.

Why imperfective here? Because the meaning is not to cheat once and finish, but rather to be cheating / to cheat in a general or ongoing sense.

A perfective present form in Polish usually has future meaning, so it would not fit well here.

Yes, Polish word order is fairly flexible, but the original order is the most neutral and natural.

Neutral:

• Przeciwnik udaje, że nie oszukuje.

Other orders are possible, but they sound marked and usually add emphasis:

• Udaje przeciwnik, że nie oszukuje.
• Że nie oszukuje, przeciwnik udaje.

Those are not the normal default choices for everyday use.

So for a learner, the best pattern to remember is:

• subject + verb + że-clause

Grammatically, przeciwnik is a masculine noun.

It usually refers to:

• a male opponent, or
• an opponent in a general sense when the noun is used in its basic dictionary form

If you want to refer specifically to a female opponent, Polish often uses:

• przeciwniczka

Examples:

• Przeciwnik udaje, że nie oszukuje.
• Przeciwniczka udaje, że nie oszukuje.

So the grammar changes with the noun chosen.

A rough English-friendly pronunciation is:

• pshe-TSIV-nik oo-DA-yeh zheh nyeh o-shoo-KOO-yeh

A few useful sound notes:

• rz in przeciwnik sounds like the s in measure or zh
• cz sounds like ch in chop
• ż / rz in że sounds like zh
• nie is roughly nyeh
• j in udaje sounds like English y
• uje in oszukuje sounds roughly like oo-yeh

If you want a more Polish-like breakdown:

• Prze-ciw-nik
• u-da-je
• że
• nie
• o-szu-ku-je

Yes. Polish present tense can be fairly broad.

Udaje can mean:

• is pretending right now
• pretends in a more general or repeated sense

So the sentence can describe:

• a current situation in a game
• a habitual pattern of behavior
• a general statement about that opponent

Context tells you which reading is intended.

No, but it is one of the most natural ways.

Other possibilities exist depending on nuance:

• Twierdzi, że nie oszukuje. = He claims that he is not cheating.
  This does not necessarily imply he is pretending.

• Zachowuje się tak, jakby nie oszukiwał. = He behaves as if he were not cheating.
  This is a bit different in structure and nuance.

• Udaje, że nie oszukuje. = He pretends that he is not cheating.
  This clearly suggests deception.

So in your sentence, udaje is a strong and very suitable choice.