Learn Hardcore Portuguese (Portugal): Mesmo quando o dia corre mal, escrever no diário ajuda-me a ver as coisas mais claramente. - Even when the day goes badly, writing in my diary helps me to see things more clearly.

With just quando o dia corre mal, the sentence is more neutral: when the day goes badly…; with mesmo quando, it highlights that the help is surprising or notable despite that.

Why is it o dia corre mal and not something like o dia vai mal or o dia é mau?

In Portuguese, correr is often used figuratively for how events unfold:

o dia corre bem – the day goes well
o dia corre mal – the day goes badly

Vai mal is possible in some contexts, but corre bem/mal is the very natural, idiomatic pair for days, meetings, exams, etc.

O dia é mau would mean the day is bad (in nature/quality), not the way the day goes turns out badly.

Why is it escrever no diário and not something like escrevendo no diário, like English writing in the journal?

Portuguese very often uses the infinitive where English uses the -ing form:

Ler ajuda. – Reading helps.
Escrever no diário ajuda-me… – Writing in the journal helps me…

The gerúndio (the -ndo form, like escrevendo) is not used in this subject position in European Portuguese.
So escrevendo no diário ajuda-me sounds wrong/unnatural in European Portuguese.

What exactly is no in no diário?

No is the contraction of the preposition em + the masculine singular article o:

em + o = no

So no diário literally is em o diário, in the journal.
You could also say no meu diário (in my journal) if you want to specify that it’s your own diary.

Why is it ajuda-me instead of me ajuda, and why is there a hyphen?

-me is an unstressed object pronoun (a clitic).
In European Portuguese, when the verb is in a simple affirmative form and not preceded by a negation or certain triggers, the pronoun normally attaches to the end of the verb with a hyphen:

ajuda-me – helps me
ajudou-me – helped me
vai ajudar-me – is going to help me

Me ajuda is typical of Brazilian Portuguese; in European Portuguese, ajuda-me is the standard form here.
The hyphen just marks that the pronoun is cliticized to the verb.

Why is there an a in ajuda-me a ver? Could I say ajuda-me ver?

The a here is a preposition that links the two verbs in this construction:

ajudar alguém a fazer alguma coisa – to help someone to do something

So: ajuda-me a ver = helps me to see.
In standard Portuguese, you need the a; ajuda-me ver without a is not correct.

Can I omit as coisas and just say ajuda-me a ver mais claramente?

Yes, you can say:

…escrever no diário ajuda-me a ver mais claramente.

That sounds natural and means helps me to see more clearly (in general).

Adding as coisas makes it slightly more concrete: to see *things more clearly*, i.e. the situations, problems, events of the day.

What’s the nuance of mais claramente? Could I say com mais clareza instead?

Mais claramente is an adverbial form: more clearly.
Com mais clareza is a prepositional phrase: with more clarity.

Both are correct and natural, and in this sentence they mean essentially the same:

…ajuda-me a ver as coisas mais claramente.
…ajuda-me a ver as coisas com mais clareza.

Mais claramente is a bit more direct and maybe slightly more informal; com mais clareza sounds a little more formal or literary, but both are common.

Why is the verb corre in the present tense, not past, if we’re talking about bad days in general?

Portuguese uses the simple present for general or habitual situations, just like English:

Quando o dia corre mal, fico cansado. – When the day goes badly, I get tired.

Here, mesmo quando o dia corre mal talks about any day that (typically) goes badly, not one specific past day.
If you said quando o dia correu mal, that would be about one specific day in the past: when the day went badly (that day).

Why is it o dia and not um dia?

Both are possible, but they suggest different things:

Mesmo quando o dia corre mal… – more like even when the day (today / a given day) goes badly…; it can sound like you’re referring to your typical day, or days in general, as a kind of concept.
Mesmo quando um dia corre mal… – even when a day goes badly…; sounds more like whenever a day happens to go badly (any day in general).

In practice, o dia is the more natural choice here when talking about your days in general or “the day” you are experiencing.

Does diário only mean journal, or does it also have other meanings?

Diário can be:

A noun:
- o diário – a diary/journal (a book where you write regularly about your life)
An adjective meaning daily:
- jornal diário – daily newspaper
- rotina diária – daily routine

In your sentence, no diário clearly uses the noun sense: in the (personal) journal.

Could I say Mesmo se o dia correr mal instead of Mesmo quando o dia corre mal?

You can, but there is a nuance:

Mesmo quando o dia corre mal… – even when the day goes badly… (describes an actual recurring situation).
Mesmo se o dia correr mal… – even if the day goes badly… (more hypothetical/conditional, like you’re imagining the possibility).

Both are grammatically fine. For a habitual, real-life pattern, mesmo quando is usually the better, more natural fit.

Can I move the clause and say Escrever no diário, mesmo quando o dia corre mal, ajuda-me…?

Yes. Both word orders are correct:

Mesmo quando o dia corre mal, escrever no diário ajuda-me…
Escrever no diário, mesmo quando o dia corre mal, ajuda-me…

The second version puts more initial focus on escrever no diário (the activity), and mesmo quando o dia corre mal becomes more of a side comment.
The meaning stays essentially the same.