Learn Hardcore Spanish (Spain): A veces leo cada comentario con tanta atención como el artículo original. - Sometimes I read each comment as carefully as the original article.

A veces leo cada comentario… = Sometimes I read every comment…
A veces yo leo cada comentario… = Sometimes I (as opposed to someone else) read every comment…

So yo is dropped here because it’s not needed for meaning and would sound slightly more marked/emphatic.

Why is it cada comentario and not todos los comentarios?

Both are possible, but they’re not identical:

cada comentario = each comment, one by one, individually.
todos los comentarios = all the comments, as a complete set.

Nuance:

leo cada comentario suggests you go through them one by one, stressing the individual comments.
leo todos los comentarios stresses that you don’t skip any; you read them all, but without the same focus on individuality.

In many contexts you could use either, but cada comentario highlights the idea of individual attention to each comment, which fits the idea of careful reading.

Why is comentario singular after cada?

In Spanish, cada is always followed by a singular noun:

cada comentario (not cada comentarios)
cada día
cada persona
cada artículo

Even though the meaning is plural (you are talking about many comments or many days), the grammar requires that cada + noun stay in the singular form.

So:

leo cada comentario = I read each comment (all of them, individually)
Using a plural noun after cada would be incorrect.

Why is there no article before cada comentario (why not el cada comentario)?

With cada, you normally do not use a definite or indefinite article:

cada comentario (not el cada comentario)
cada persona
cada día de la semana

This is just how cada works in Spanish:
cada + singular noun (no article).

If you want an article, you must change the structure:

cada uno de los comentarios = each one of the comments
Here uno takes de + article, not cada directly:
cada uno de los comentarios (correct)
cada los comentarios (incorrect)

What does con tanta atención como mean, grammatically speaking?

This is a standard equality comparison structure with a noun:

tanto / tanta / tantos / tantas + noun + como

It means as much [noun] as.

Here:

tanta agrees with atención (feminine singular).
con tanta atención como el artículo original = with as much attention as (I give to) the original article.

Other examples:

Tengo tanto trabajo como tú. = I have as much work as you.
Bebió tanta agua como yo. = He/She drank as much water as I did.
Hay tantas personas como ayer. = There are as many people as yesterday.

Why is it tanta and not tanto or tan?

You have two different patterns:

tan + adjective/adverb + como
- tan rápido como = as fast as
- tan atento como = as attentive as
tanto / tanta / tantos / tantas + noun + como
- tanta atención como = as much attention as
- tantos libros como = as many books as

In the sentence we are comparing the amount of attention (a noun: atención), so we must use tanta:

atención is feminine singular → tanta atención

Using tan atención or tanto atención would be incorrect.

Could I say tan atentamente como el artículo original instead of con tanta atención como el artículo original?

You can, and it is grammatically correct, but it slightly changes the style and focus.

con tanta atención (noun: attention)
Focus: the amount of attention given.
tan atentamente (adverb: attentively)
Focus: the manner of reading (how attentively you read).

So:

A veces leo cada comentario con tanta atención como el artículo original.
= Sometimes I read each comment with as much attention as (I read) the original article.
A veces leo cada comentario tan atentamente como el artículo original.
= Sometimes I read each comment as attentively as (I read) the original article.

Both are natural; the original with con tanta atención is very common and maybe a bit more neutral-sounding.

Why is it el artículo original and not al artículo original?

al = a + el, and you use it when a verb or expression needs the preposition a:

Voy al cine. = I go to the cinema.
Miro al hombre. (direct object with a person → personal a)

In the sentence:

con tanta atención como el artículo original

The comparison is:

leo cada comentario con tanta atención
leo el artículo original con la misma atención

The verb involved is leer (to read). Leer does not normally take the preposition a with non-human direct objects:

Leo el artículo. (correct)
Leo al artículo. (incorrect; sounds very odd)

Since you would say leo el artículo original, you must also say como el artículo original, not como al artículo original.

Is con atención a fixed expression, or could I drop con and just say leo atención?

Con atención is a very common, almost fixed expression meaning carefully / attentively.

Structure:

leer / escuchar / mirar / estudiar + con atención

You cannot drop con here:

leo atención is incorrect.
You need something like:
- leo con atención (I read carefully)
- leo atentamente (I read attentively)

So in the original:

leo cada comentario con tanta atención
= I read each comment with so much attention / so carefully.

Can I say A veces leo todos los comentarios con tanta atención como el artículo original instead? What would change?

Yes, that sentence is perfectly correct. The differences:

cada comentario = each comment, individually; emphasizes the idea of going one by one.
todos los comentarios = all the comments; emphasizes completeness (you don’t miss any).

So:

A veces leo cada comentario…
suggests patience and attention to each single comment.
A veces leo todos los comentarios…
tells us you read all of them, but doesn’t highlight the individual, one-by-one idea as strongly.

Both would be understood as you taking the comments seriously; the original version puts more emphasis on the individual comments.

Could the word order be Leo cada comentario con tanta atención como el artículo original a veces?

That word order is grammatically possible but sounds unnatural in modern Spanish.

Most natural placements for a veces here are:

A veces leo cada comentario con tanta atención como el artículo original.
Leo a veces cada comentario con tanta atención como el artículo original. (less common, but still acceptable)

Placing a veces at the very end:

Leo cada comentario con tanta atención como el artículo original a veces.

sounds marked or awkward in standard Spanish. It’s better to keep a veces near the beginning of the clause.

In Spain, is artículo the usual word here, or could people say something else for an online text?

In Spain, artículo is very normal if you’re talking about something like a news article, blog article, or opinion piece.

Other words you might hear, depending on the context and platform:

entrada (more like blog post or a post on a website)
publicación (a post, publication)
post (an English borrowing; common in informal internet talk)

But in a neutral sentence like this, artículo is completely standard and widely used in Spain.