Min bror brukar kommentera varje inlägg som jag skriver om resan.

In Swedish, a possessive word like min usually replaces the article, and the noun stays in the indefinite form.

So:

• min bror = my brother
• not min brodern
• not den min bror

This is very common in Swedish:

• min bok = my book
• mitt hus = my house
• mina vänner = my friends

So min bror is exactly the normal way to say my brother.

Swedish possessives change depending on the noun.

• min is used with a singular common-gender noun
• mitt is used with a singular neuter noun
• mina is used with plurals

Here, bror is a singular common-gender noun, so you use min.

Compare:

• min bror = my brother
• mitt inlägg = my post
• mina bröder = my brothers

Brukar shows a habit or something that happens usually / regularly / tends to happen.

So Min bror brukar kommentera ... means something like:

• my brother usually comments ...
• my brother tends to comment ...
• my brother habitually comments ...

It is not about one single event. It describes what he generally does.

Because brukar is followed directly by the infinitive form of the next verb, without att.

So:

• brukar kommentera
• brukar skriva
• brukar läsa

Not:

• brukar att kommentera

This is similar to some other verb combinations in Swedish where att is not used.

That is a very common difference between English and Swedish.

In standard Swedish, kommentera is usually used directly with an object:

• kommentera ett inlägg = comment on a post
• kommentera artikeln = comment on the article

So kommentera varje inlägg is normal Swedish.

English needs on, but Swedish usually does not here.

Because varje works like English every: it is followed by a singular noun.

So:

• varje inlägg = every post
• varje dag = every day
• varje gång = every time

If you wanted a plural idea like all posts, you would say:

• alla inlägg

So:

• varje inlägg = every single post
• alla inlägg = all posts

Som introduces a relative clause. It means that, which, or sometimes who, depending on context.

Here:

• varje inlägg som jag skriver om resan

means:

• every post that I write about the trip

So som jag skriver om resan describes inlägg.

A useful point: Swedish often uses som where English might choose that, which, or who.

Because after som, you are inside a subordinate clause / relative clause, and Swedish word order changes.

In a main clause, Swedish often has verb-second word order.
But in a subordinate clause, the subject usually comes before the verb:

• jag skriver = I write
• som jag skriver = that I write

So som jag skriver om resan is the normal order.

A useful comparison:

• Main clause: Jag skriver om resan.
• Relative clause: ... inlägg som jag skriver om resan

Yes, om resan means about the trip.

It connects most directly to skriver:

• jag skriver om resan = I write about the trip

So the full phrase means:

• every post that I write about the trip

In other words, these are posts whose topic is the trip.

Resan is the definite form, meaning the trip.

So:

• resa = trip
• en resa = a trip
• resan = the trip

The sentence uses resan because it refers to a specific trip that is already known from the context.

If you said om en resa, that would sound more like about a trip in a more general or not-yet-identified sense.

It can sometimes be a little tricky, because skriva om can have more than one meaning depending on context.

Very often:

• skriva om något = write about something

But in some contexts, Swedish skriva om can also mean rewrite.

Here, though, the meaning is clearly write about, because om resan naturally means about the trip.

So in this sentence:

• jag skriver om resan = I write about the trip

not I rewrite the trip.