Learn Hardcore Russian: В этой социальной сети я редко читаю комментарии под постами. - In this social network I rarely read the comments under posts.

With websites, platforms, social networks, Russian very often uses «в» (in), treating them as a kind of “space” you are inside:
- в этой социальной сети – in this social network
- в Фейсбуке, в Инстаграме, в Твиттере
«на» is used with some platforms too (e.g. на YouTube, на форуме), but for “social network” as a general noun, «в социальной сети» is the normal choice.

So the Russian pattern is: “in this social network”, not “on this social network”, even though English says “on.”

What case is «этой социальной сети», and why is it used here?

The phrase «этой социальной сети» is in the prepositional case (feminine singular).

Breakdown:

эта → этой (Prepositional, fem. sg.)
социальная → социальной (Prepositional, fem. sg.)
сеть → сети (Prepositional, fem. sg.)

We use the prepositional case after the preposition «в» when we talk about location (where?):

Где? – В этой социальной сети.
I rarely read comments in this social network.

So: «в» + prepositional = “in/at/on” (location).

What case is «комментарии», and how can I tell?

«Комментарии» here is in the accusative plural, because it is the direct object of the verb «читаю»:

(Я) читаю что? комментарии – I read what? comments.

For inanimate masculine and neuter nouns, the accusative plural form is identical to the nominative plural:

Nom. pl. комментарии – Acc. pl. комментарии

So it looks like nominative plural, but its function is accusative (direct object).

Why is it «под постами» and not something like «под постах» or «под посты»?

The preposition «под» can take two cases:

Instrumental – for location (where something is):
- Книга под столом. – The book is under the table.
Accusative – for direction (motion to a place):
- Я кладу книгу под стол. – I put the book under the table.

In the sentence, comments are located under posts, not moving somewhere:

Комментарии под постами. – Comments under the posts.

So «под» + instrumental plural gives us:

пост → постами (Instr. pl.) → под постами.

Could you also say «комментарии к постам» instead of «комментарии под постами»? Is there a difference?

Yes, you can say «комментарии к постам», but the nuance changes slightly:

комментарии под постами – literally comments under the posts, focusing on where they appear on the page (visually under each post).
комментарии к постам – comments to the posts, focusing more on the relationship: comments addressed to / attached to those posts.

In practice, both are possible; «под постами» emphasizes the layout/location, which fits well with a website or social feed.

Why is «редко» placed before the verb: «я редко читаю»? Could I say «я читаю редко»?

Both are grammatically correct, but the usual neutral word order for frequency adverbs is:

Subject + adverb of frequency + verb
→ Я редко читаю комментарии…

If you say «я читаю редко», it’s still correct, but it sounds:

a bit more emphatic or contrastive, like stressing how rarely you read in general, or
a bit less natural in this specific sentence than the standard «я редко читаю комментарии…».

So:

Most natural here: Я редко читаю комментарии под постами.

Why is the verb «читаю» (imperfective present) used, and not a perfective form like «прочитаю»?

The sentence describes a regular habit or typical behavior:

I rarely read comments (as a general tendency).

For habits, repeated actions, and general statements, Russian uses the imperfective aspect in the present:

Я редко читаю комментарии. – I rarely read comments.

A perfective form like «прочитаю» would describe a single completed action in the future:

Я редко прочитаю все комментарии. – I will rarely manage to read all the comments (sounds odd by itself and needs more context).

So «читаю» is correct here because we’re talking about how you usually behave, not about completing one specific act of reading.

How flexible is the word order? Could I say «Я редко читаю комментарии в этой социальной сети под постами»?

Russian word order is fairly flexible, and your version is grammatically correct, but the original order is more natural and clear:

В этой социальной сети я редко читаю комментарии под постами.

Putting «в этой социальной сети» first sets the context: “In this social network…” Then we get the statement about your behavior there.

If you say:

Я редко читаю комментарии в этой социальной сети под постами.

it’s still understandable, but:

the phrase becomes heavier and less elegant, and
«в этой социальной сети» is more tightly attached to «комментарии» than to the whole situation.

So the given word order nicely emphasizes: In this particular social network, I rarely do this.

What exactly is the difference between «социальной сети» and the shorter word «соцсети» that I see online?

«социальная сеть» – the full, formal phrase: social network.
«соцсеть» – a colloquial shortened form often used in speech and informal writing (like “socials” or “social media” in English).

In your sentence, a more casual version could be:

В этой соцсети я редко читаю комментарии под постами.

Meaning is the same; it just sounds more informal / conversational.

How do you pronounce the tricky words «социальной», «сети», and «комментарии»? Where is the stress?

Approximate pronunciation with stressed syllables in bold:

социа́льной – sa-tsi-AL-noy
(stress on а: социа́льной)
сети́ – si-TEE
(stress on и: сети́)
коммента́рии – ka-men-TA-ree-i
(stress on а: коммента́рии)

So the phrase «в этой социальной сети» sounds like:

v E-toy sa-tsi-AL-noy si-TEE
and «комментарии под постами» like:
ka-men-TA-ree-i pad pa-STAH-mee (with stress on ТА in комментарии and on по́ст in постами).