Learn Hardcore Portuguese (Portugal): No fim de cada semana, escrevo uma pequena conclusão sobre o que aprendi. - At the end of each week, I write a short conclusion about what I have learned.

em + o fim → no fim (at the end)
If the noun were feminine, you would get na = em + a (e.g. na semana = in the week), but fim is masculine, so you must use no, not na.

You almost never say em fim de; in this expression you use the contracted form no fim de.

What is the difference between fim de semana and fim de cada semana?

In European Portuguese:

o fim de semana = the weekend (a fixed expression; usually Saturday and Sunday)
no fim de semana = on/at the weekend

But:

no fim de cada semana = at the end of each week

So fim de semana is a specific phrase meaning weekend, while fim de cada semana is a more literal end of each week, which could be understood as sometime around the week’s end, not necessarily exactly Saturday–Sunday as a fixed time name.

Why do we say de cada semana? Could I say something like todas as semanas or semanalmente instead?

Yes, there are alternatives with slightly different nuances:

No fim de cada semana = At the end of each week
Emphasises each individual week, one by one.
Todas as semanas, escrevo… = Every week, I write…
More neutral; focuses on frequency, not necessarily on the end of the week.
Escrevo… semanalmente. = I write… weekly.
More formal/abstract, sounds less like a personal routine and more like a scheduled task.

You choose based on what you want to emphasise:

End of the week: no fim de cada semana
Just frequency: todas as semanas / semanalmente

Is the comma after semana necessary? Could I say the sentence without it?

Both are correct:

No fim de cada semana, escrevo uma pequena conclusão…
No fim de cada semana escrevo uma pequena conclusão…

The initial phrase No fim de cada semana is an adverbial time expression. In European Portuguese, you often use a comma after such an introductory phrase, especially in writing, but it’s not strictly obligatory in a short sentence like this.

You could also put the time expression at the end, usually without a comma:

Escrevo uma pequena conclusão no fim de cada semana.

What tense is escrevo, and why is it used here?

Escrevo is the present indicative (1st person singular).

Here, the present tense is used for a habitual action or routine:

Escrevo uma pequena conclusão…
= I (usually) write / I write (as a habit) a small conclusion…

This is parallel to English I write when talking about repeated actions:

Every week, I write a short summary.

Do I need to say eu escrevo, or is escrevo enough?

In European Portuguese, the subject pronoun is usually omitted because the verb ending already shows the subject:

(Eu) escrevo = I write
(Tu) escreves = you write (informal singular)
(Ele/Ela) escreve = he/she writes

So:

Escrevo uma pequena conclusão… is completely natural and normal.
Eu escrevo uma pequena conclusão… is also correct, but you would usually add eu only to:
- Contrast with someone else: Eu escrevo, tu lês.
- Emphasise the subject: Eu escrevo, não ele.

Why is it uma pequena conclusão and not uma conclusão pequena? Does the position of pequena change the meaning?

Adjectives in Portuguese can often go before or after the noun, with subtle differences in style or nuance.

uma pequena conclusão
- Slightly more idiomatic here.
- Often sounds a bit more subjective or evaluative: a modest, short conclusion.
uma conclusão pequena
- Grammatically correct, but sounds more literal or descriptive (you’re describing its physical or quantitative size).

In many fixed or common expressions, quality or size adjectives like bom, mau, grande, pequeno often come before the noun, especially in everyday language:

um grande amigo (a very good friend / close friend)
uma pequena ajuda (a little help)

Why uma pequena and not um pequeno? What is the gender of conclusão?

Conclusão is a feminine noun:

a conclusão = the conclusion
uma conclusão = a conclusion

Adjectives and articles must agree in gender and number with the noun:

Feminine singular:
- uma pequena conclusão
- a pequena conclusão
Masculine singular:
- um pequeno texto (a small text)
- o pequeno texto

So we say uma pequena conclusão because conclusão is feminine.

What does sobre mean here, and how is it different from de or acerca de?

In this sentence, sobre means about / regarding:

uma pequena conclusão sobre o que aprendi
= a small conclusion about what I learned

Some related options:

sobre
Very common and neutral: um livro sobre história (a book about history).
acerca de
More formal; often used in writing: um relatório acerca de…
de
Can also mean about, but sounds different:
- uma conclusão do que aprendi
  = a conclusion of what I learned (the conclusion comes from what I learned; slightly different nuance).

In everyday speech, sobre is the most natural for about in this context.

Why do we say sobre o que aprendi and not just sobre que aprendi?

O que is a fixed combination meaning roughly what or that which:

o que aprendi = what I learned / that which I learned

The structure is:

sobre (about)
o que (what / that which)
aprendi (I learned)

Que alone is usually a relative pronoun that needs a specific noun before it:

a coisa que aprendi = the thing that I learned

So:

sobre o que aprendi = about what I learned
sobre que aprendi would be wrong here, because que doesn’t have an antecedent noun to refer to.

You could also say:

sobre aquilo que aprendi = about that which I learned
(a bit heavier/more formal).

Why is aprendi (past tense) used, and not aprendo or tenho aprendido?

Aprendi is the pretérito perfeito (simple past), 1st person singular:

aprendi = I learned / I have learned (completed action)

In this sentence, the idea is:

During the week, I learn new things.
By the end of that week, my learning for that period is complete.
Then I write a conclusion about what I have learned in that week → o que aprendi.

So aprendi matches a completed period (the week) at the moment you’re writing the conclusion.

Other tenses would change the meaning:

sobre o que aprendo = about what I learn (in general, ongoing)
sobre o que tenho aprendido = about what I have been learning (recent ongoing process, more like English have been learning)

Here, aprendi underlines that, each week, you sum up the learning that is now finished for that week.

Can I change the word order and say Escrevo uma pequena conclusão no fim de cada semana? Is there any difference?

Yes, that is perfectly correct:

No fim de cada semana, escrevo uma pequena conclusão…
Escrevo uma pequena conclusão no fim de cada semana.

Both mean the same. The differences are mostly stylistic:

Starting with No fim de cada semana slightly emphasises the time of the action.
Putting no fim de cada semana at the end sounds a bit more neutral, like typical spoken word order.

In both cases, it is clear that the action is habitual and connected to the end of each week.

Why cada semana without an article? Can we say cada a semana?

You never use an article directly after cada.

Correct:

cada semana = each week
cada dia = each day
cada aluno = each student

Incorrect:

cada a semana ✗
cada o dia ✗

Cada already has the meaning of each / every, so you don’t add o/a/os/as after it. If you want an article, you normally switch to another structure, e.g. todas as semanas (every week).

Can I say No final de cada semana instead of No fim de cada semana? Is there a difference?

Yes, you can say both:

No fim de cada semana…
No final de cada semana…

They are very close in meaning. In many contexts, they are interchangeable. Small nuance:

fim is shorter and very common in everyday speech.
final can sound a bit more formal or slightly more technical, depending on context, but here it would still be perfectly natural.

So you can safely use either in this sentence.