Learn Hardcore Portuguese (Portugal): É difícil medir a qualidade de um dia só pelo que correu mal. - It is hard to measure the quality of a day just by what went wrong.

In English we use a “dummy” subject (it), but in Portuguese that it is simply not expressed.
You could say Isso é difícil (That is difficult), but that refers to some specific thing already mentioned, not to the action of measuring a day’s quality in general.

So:

É difícil medir a qualidade de um dia... = general truth, impersonal
Isso é difícil. = that particular thing is difficult

Why is it É difícil and not Está difícil?

Ser (é) is used for characteristics that are seen as inherent, general, or permanent, while estar (está) is used for temporary states or conditions.

Here, the idea is that, in general, measuring the quality of a day only by what went wrong is inherently difficult. So é is natural:

É difícil medir a qualidade de um dia...
It is (in general) difficult to measure the quality of a day...

If you said Está difícil medir..., it would sound like:

Right now, in this particular situation, it is (temporarily) difficult to measure...

That would feel oddly specific and less natural in this generic, reflective sentence.

Why is the verb medir in the infinitive here? Could it be É difícil de medir?

The structure é + adjective + infinitive is very common:

É difícil medir...
É fácil entender...
É importante saber...

It expresses “doing this action is [adjective]”.

You can sometimes say é difícil de medir, but:

É difícil medir a qualidade de um dia...
is smoother, more standard, and more idiomatic here.
É difícil de medir tends to be used more when there is an explicit noun that the adjective refers to:
Esta grandeza é difícil de medir – This quantity is difficult to measure.

In your sentence, the focus is on the action itself, so the bare infinitive medir after é difícil is the most natural choice.

Can I change the word order to Medir a qualidade de um dia só pelo que correu mal é difícil?

Yes. That is grammatically correct and means the same thing.

Two natural options are:

É difícil medir a qualidade de um dia só pelo que correu mal.
Medir a qualidade de um dia só pelo que correu mal é difícil.

The first puts the comment (it’s difficult) upfront, which is very typical in spoken language.
The second starts with the action (measuring the quality...), which can sound a bit more formal or reflective, but is still very normal.

So this is a matter of style and emphasis, not grammar.

Why is it a qualidade de um dia and not a qualidade do dia?

Both forms are possible, but the nuance changes:

a qualidade de um dia
– literally the quality of a day
– generic: we’re talking about any day in general, the idea of “a day”.
a qualidade do dia
– the quality of the day
– more specific: it tends to sound like a particular, identified day (e.g. today, that day).

Since the sentence is expressing a general reflection about days in life, um dia sounds more natural.
If the speaker were clearly talking about one specific day, do dia could fit that context.

What exactly does só mean here, and how is it different from apenas or somente?

In this sentence, só means “only / just”:

só pelo que correu mal
= only by what went wrong

Differences:

só – very common and colloquial, used in both speech and writing.
apenas – a bit more formal / neutral, frequent in writing.
somente – also more formal, often found in writing.

All three can work here:

...só pelo que correu mal.
...apenas pelo que correu mal.
...somente pelo que correu mal.

In everyday European Portuguese speech, só is by far the most common choice.

Why is só placed before pelo and not elsewhere, like after the verb?

Só usually appears right before the word or phrase it limits:

só pelo que correu mal
= only by what went wrong
(not by what went well or other factors)

If you move só, you change either the emphasis or the meaning:

É só difícil medir a qualidade de um dia pelo que correu mal.
Now só is limiting difícil → It is only difficult (and nothing else) – odd and confusing here.
É difícil medir só a qualidade de um dia pelo que correu mal.
This sounds like you are only measuring the quality of a day, as opposed to something else, which is not the intended idea.

So só is correctly placed to show that the criterion (what went wrong) is what is being restricted.

What is pelo, and why not por o or another preposition?

Pelo is a contraction of por + o:

por + o = pelo
por + a = pela
por + os = pelos
por + as = pelas

Here the structure is:

pelo que correu mal = por + o que correu mal

The preposition por in this context means “by / based on / using as a criterion”:

medir... pelo que correu mal
= to measure (it) by what went wrong

Using para or de would be incorrect here:

para o que correu mal – for what went wrong (purpose/goal) → wrong meaning
do que correu mal – of what went wrong (possession/origin) → wrong meaning

So por is the correct preposition, and it must contract with the masculine singular article o → pelo.

What does o que mean here, and how is it different from que on its own?

In pelo que correu mal, the phrase o que functions as a neutral pronoun meaning “what / the things that”.

o – neutral article
que – relative pronoun
o que – that which / what / the things that

So pelo que correu mal is roughly:

pelo que correu mal
→ by what went wrong / by the things that went wrong

If you used que alone here (e.g. pelo que correu mal without the o), it would no longer be a complete noun phrase and would sound wrong. You need o que to form a substantive clause (“that which went wrong”).

What does the expression correu mal literally mean, and how is it used?

Literally:

correr – to run
mal – badly

But in this expression, correr mal is an idiom meaning “to go wrong / to turn out badly”:

A reunião correu mal. – The meeting went badly / went wrong.
Tudo correu mal ontem. – Everything went wrong yesterday.

In your sentence:

o que correu mal
→ what went wrong / the things that went badly

In European Portuguese, correr mal is a very common way to say “to go wrong”.

Why is correu in the past tense? Could it be corre mal instead?

Correu is the pretérito perfeito (simple past), used for finished events:

pelo que correu mal
= by what went wrong (earlier that day, already happened)

The idea is: you’re judging the quality of a day after it has happened, based on the things that already went wrong.

Corre mal would be present tense:

o que corre mal – what goes wrong (regularly, habitually)

That would change the meaning to something more like:

It is difficult to measure the quality of a day just by what (always) goes wrong.

For a single, completed day, correu mal (past) is the natural choice.

Could I replace o que correu mal with as coisas que correram mal? Is there a difference?

Yes, grammatically you can:

só pelo que correu mal
só pelas coisas que correram mal

Both mean roughly “only by the things that went wrong”.

Differences:

o que correu mal is more compact and abstract – “what went wrong” in a general, non-countable sense.
as coisas que correram mal is more concrete – literally “the things that went wrong”.

The original version with o que correu mal feels more natural and elegant in a reflective, philosophical sentence like this.

Would the sentence still be correct if we replaced mal with bem, like pelo que correu bem?

Yes, grammatically it’s fine:

É difícil medir a qualidade de um dia só pelo que correu bem.
= It’s hard to measure the quality of a day just by what went well.

The structure stays exactly the same:

o que correu mal – what went wrong
o que correu bem – what went well

Only the meaning changes: now you are saying we also shouldn’t judge a day only by its positive events.

Would Brazilian Portuguese speakers say this sentence the same way?

They would understand it perfectly, and it is correct in Brazil too, but some expressions differ in what feels most natural:

In European Portuguese, correr mal is very common for “to go wrong”.
In Brazilian Portuguese, people often say:
- dar errado – to go wrong
  - É difícil medir a qualidade de um dia só pelo que deu errado.

So, while É difícil medir a qualidade de um dia só pelo que correu mal is fully correct and understandable in Brazil, correr mal sounds especially typical of Portuguese from Portugal.