No merece la pena discutir por eso.

Merecer la pena is a very common Spanish expression meaning to be worth it or to be worthwhile.

So in this sentence, No merece la pena... means It isn’t worth... / It’s not worth it to...

A useful thing to remember is that this is an idiomatic chunk. Learners often understand it best if they memorize merecer la pena as one expression, rather than translating each word separately every time.

Because the fixed expression is merecer la pena, not just merecer pena.

Here, pena does not mean penalty in the usual legal sense. In this idiom, the whole phrase means something like to be worth the trouble / effort / pain.

So:

• merece la pena = it’s worth it
• no merece la pena = it’s not worth it

It’s best to learn la pena as part of the set phrase.

Because after expressions like merece la pena, Spanish often uses an infinitive to say what action is or isn’t worth doing.

So:

• Merece la pena verlo. = It’s worth seeing it.
• No merece la pena esperar. = It’s not worth waiting.
• No merece la pena discutir por eso. = It’s not worth arguing about that.

This works a lot like English it’s worth doing.

In this sentence, discutir means to argue.

This is an important false-friend-type issue for English speakers:

• English discuss = talk about something
• Spanish discutir often = argue, quarrel, or have a disagreement

Sometimes discutir can mean discuss, depending on context, but very often it has a stronger sense than English discuss.

So here, No merece la pena discutir por eso is naturally understood as:

• It’s not worth arguing about that

If you want the calmer idea of discuss, Spanish often uses things like:

• hablar de
• comentar
• debatir (more formal / structured)

Here, por eso means about that, over that, or because of that, depending on how naturally you translate it in English.

In this sentence, the idea is:

• discutir por eso = to argue over that / about that

The preposition por often gives the sense of because of, over, or on account of.

So the sentence is basically saying:

• That isn’t a good enough reason to argue
• It’s not worth arguing over that

Because por and para do different jobs.

Here, por eso means something like:

• because of that
• over that
• about that

By contrast, para eso usually means:

• for that
• for that purpose

Compare:

• Discutir por eso = to argue about/over that
• Hice eso para ayudarte = I did that to help you

So por eso is the natural choice in this sentence because it refers to the reason/cause/topic of the argument.

Spanish often leaves out the subject when it is understood or when the sentence is being used impersonally.

In No merece la pena discutir por eso, the idea is similar to English:

• It’s not worth arguing about that

English uses a dummy subject: it.
Spanish does not need that kind of subject in the same way.

You can think of the structure as:

• [Discutir por eso] no merece la pena
• Arguing about that is not worth it

But Spanish very naturally prefers:

• No merece la pena discutir por eso

So the sentence is normal and complete even without an explicit subject.

Yes, but the original order is the most natural and common.

Most natural:

• No merece la pena discutir por eso.

Possible, but less neutral:

• Discutir por eso no merece la pena.

Both are grammatical. The second version puts more focus on discutir por eso. The first version is what you are more likely to hear in everyday speech.

Yes. No vale la pena discutir por eso is also correct and very common.

Both mean basically the same thing:

• No merece la pena...
• No vale la pena...

A useful regional note:

• Merece la pena is especially common and very natural in Spain.
• Vale la pena is also widely used, and many learners will hear it a lot in Latin America too.

So if you are learning Spanish from Spain, no merece la pena is an excellent expression to remember.

It is neutral everyday Spanish.

It works in many situations:

• speaking to a friend
• giving advice
• commenting on a situation
• writing informal or semi-neutral Spanish

It doesn’t sound slangy, and it doesn’t sound especially formal either. It’s just a very natural, common phrase.

Yes, very easily. Eso just stands for that. You can replace it with a noun or another phrase.

For example:

• No merece la pena discutir por dinero. = It’s not worth arguing over money.
• No merece la pena discutir por una tontería. = It’s not worth arguing over something silly.
• No merece la pena discutir por este tema. = It’s not worth arguing about this topic.

So once you know the pattern, you can reuse it in many situations:

• No merece la pena + infinitive + por/sobre...

A native speaker would often say it to calm a situation down or to advise someone not to continue an argument.

Typical situations:

• two people are fighting over something minor
• someone is getting upset about a small issue
• you want to say let it go

It often carries the tone:

• This is too small to argue about
• Let’s not make this into a bigger problem
• It’s not worth the hassle

So it is not just grammatically useful; it is also a very practical everyday phrase.