Posso blefar, mas não posso trapacear.

Posso is the first-person singular present tense of poder.

So:

• eu posso = I can / I may
• infinitive: poder

In this sentence, posso is used with another verb in the infinitive:

• posso blefar
• não posso trapacear

Portuguese often drops the subject pronoun when it is clear from the verb ending.

Because posso already means I can, native speakers do not need to say eu here.

So both are possible:

• Posso blefar, mas não posso trapacear.
• Eu posso blefar, mas não posso trapacear.

Adding eu usually gives a little more emphasis or contrast.

After poder, the next verb normally stays in the infinitive.

That is the normal pattern:

• posso blefar
• posso trapacear
• posso sair
• posso falar

This is very similar to English can + base verb:

• can bluff
• can cheat

They are not the same thing.

• blefar = to bluff; to pretend you are stronger, more confident, or in a better position than you really are
• trapacear = to cheat; to break the rules unfairly

So blefar can be part of the game, especially in card games like poker.
Trapacear means going beyond that and actually cheating.

That contrast is the key point of the sentence.

Yes. Blefar is a real and common verb in Brazilian Portuguese, especially in contexts like:

• poker
• games
• negotiation
• figurative speech

It is a borrowed word, but it behaves like a normal -ar verb in Portuguese.

For example:

• eu blefo
• você blefa
• ele blefou

So even though it may look foreign to an English speaker, it is perfectly normal Portuguese.

In standard Portuguese, não usually goes before the conjugated verb:

• não posso
• não quero
• não sei

So não posso trapacear is the normal way to say I cannot / may not cheat.

You may hear posso não in Brazilian speech, but that usually has a different nuance, such as:

• maybe I won’t
• I might not
• a softer, more colloquial refusal

So in this sentence, não posso is the correct standard structure.

Yes, Posso blefar, mas não trapacear is understandable, and some people might say it.

But repeating posso:

• makes the sentence clearer
• sounds more balanced
• is the safest, most natural full version

So:

• Posso blefar, mas não posso trapacear. = full and clear
• Posso blefar, mas não trapacear. = shorter, more elliptical

A learner should usually prefer the full version first.

Because mas means but and introduces a contrast.

In Portuguese, it is standard to put a comma before mas when it links two contrasting clauses:

• Posso blefar, mas não posso trapacear.

That comma is the expected punctuation here.

A rough Brazilian Portuguese pronunciation is:

POH-so bleh-FAR, maz NÃW POH-so tra-pa-se-AR

A few useful notes:

• posso: the ss sounds like a normal English s
• mas: often sounds like maz in connected speech
• não: nasal sound; this is one of the trickier words for English speakers
• trapacear: the ce sounds like seh
• the stress in blefar is on the last syllable: ble-FAR
• the stress in trapacear is also on the last syllable: tra-pa-se-AR

It can mean either, depending on context.

In Portuguese, poder often covers:

• ability: can
• permission: may / be allowed to

So this sentence could mean:

• I am able to bluff, but I cannot cheat
• I’m allowed to bluff, but I’m not allowed to cheat

Usually the situation tells you which meaning is intended.

Trapacear is fairly neutral and common. It works well in everyday speech and is easy to understand.

It means cheating in a broad sense:

• in games
• in competitions
• in situations where someone breaks the rules unfairly

So it is a good general verb for to cheat.