Learn Hardcore Russian: Этот стрим оказался интересным. - This stream turned out to be interesting.

So интересным is masculine singular instrumental, agreeing with стрим.
Стрим оказался интересным is the grammatically normal form; Стрим оказался интересный sounds wrong to native speakers in this structure.

What nuance does оказался add compared to just saying был?

Оказался suggests “turned out to be / ended up being (contrary to expectation or after checking)”.

Compare:

Этот стрим был интересным. – This stream was interesting. (neutral statement of fact)
Этот стрим оказался интересным. – This stream turned out to be interesting.
→ implies some element of surprise, discovery, or at least that the speaker didn’t fully know in advance.

So оказался adds the idea of a result that became clear over time or after experience, not just a static description.

What exactly is оказался grammatically (tense, aspect, person, etc.)?

Оказался is:

verb: оказаться
aspect: perfective (completed result)
tense: past
person: (past tense doesn’t show person in Russian, only gender/number)
gender/number: masculine singular
reflexive: yes, has the reflexive ending -ся

So it literally means “(he/it) turned out / proved to be” referring to a completed outcome in the past.

Why does the verb end in -ся? What does that mean here?

The -ся (or -сь) ending marks a reflexive / middle verb form.
The base is оказать, but оказаться is a separate verb with its own meaning.

оказать помощь – to give/provide help (non‑reflexive)
оказаться интересным – to turn out to be interesting (reflexive/middle)

In many verbs like this, -ся does not mean literal self‑action; it often makes the verb more “inward” or “result-oriented”, like to end up, to find oneself, to turn out.

What gender is стрим and how does it decline?

Стрим is a masculine, inanimate noun borrowed from English stream (in the sense of livestream).

It declines like a regular masculine noun ending in a consonant:

Nominative: стрим – stream (subject)
Genitive: стрима
Dative: стриму
Accusative: стрим
Instrumental: стримом
Prepositional: о стриме

Plural:

Nom.: стримы
Gen.: стримов, etc.

In the sentence Этот стрим оказался интересным, стрим is nominative singular masculine.

Why is the pronoun этот used, and what form is it?

Этот is the masculine singular nominative form of the demonstrative pronoun этот / эта / это / эти (this / these).

Here:

этот – masculine, nominative (agrees with стрим)
стрим – masculine, nominative (subject)

So Этот стрим means “this stream” and signals a specific, identifiable stream (for example, the one you and the listener know or have just watched).

Can we drop этот and just say Стрим оказался интересным?

Yes, you can.

Этот стрим оказался интересным. – This stream turned out to be interesting. (a particular one you’re pointing to in context)
Стрим оказался интересным. – The stream turned out to be interesting. (context will show which one; sounds a bit more general or like “the stream we’re talking about”.)

Both are grammatical; этот simply makes it explicitly “this (specific) one”.

Could we instead say Этот стрим был интересным? Is there a difference?

Yes, you can say Этот стрим был интересным, and it is correct.

Difference in nuance:

был интересным – a simple past description: it was interesting, no extra implication.
оказался интересным – it turned out interesting; you’re reporting a conclusion or perhaps mild surprise.

So оказался focuses on the outcome of an evaluation; был is more neutral and static.

Is there a version with a short-form adjective, like оказался интересен? How is that different?

Yes, you can say:

Этот стрим оказался интересен.

This uses the short-form adjective интересен instead of the full-form instrumental интересным.

Nuance:

оказался интересным – very standard, neutral, commonly used.
оказался интересен – feels a bit more bookish or stylistically elevated; it often emphasizes the state or quality itself.

Both are grammatically correct; in everyday speech, оказался интересным is more frequent.

Can the word order be changed, like Этот стрим интересным оказался?

Russian word order is flexible, so other orders are possible, but not all sound equally natural here.

Этот стрим оказался интересным. – most natural, neutral.
Этот стрим интересным оказался. – possible, but more marked; can sound poetic, expressive, or odd in casual speech; it puts some emphasis on интересным.
Стрим этот оказался интересным. – also possible; adds emphasis or contrast to этот (as opposed to some other stream).

For everyday neutral speech, Этот стрим оказался интересным is the best choice.

How would I say “This stream turned out to be boring” instead?

You simply change the adjective:

Этот стрим оказался скучным.

Here скучным (boring) is also masculine singular instrumental, agreeing with стрим, just like интересным.

How do I negate this? For example, “This stream turned out not to be interesting.”

Use не with the adjective in the instrumental:

Этот стрим оказался неинтересным. – This stream turned out to be not interesting / boring.

Here:

неинтересным is written together (common with adjectives where не- forms a natural opposite: интересный → неинтересный).
It stays in masculine singular instrumental to agree with стрим and follow оказался.

How is the whole sentence pronounced and where is the stress?

Approximate pronunciation with stresses:

Э́тот – [É-tat]
стрим – [strim]
оказа́лся – [ɐ-kɐ-ZÁL-sʲə]
интересным – [in-tʲɪ-RÉS-nym]

So, putting it together (main stressed syllables in caps):

É-tat STRIM ɐ-kɐ-ZÁL-sʲə in-tʲɪ-RÉS-nym.

Primary stresses:

Э́тот, оказа́лся, интере́сным.