Learn Hardcore Finnish: Tämä sarja on hauska, mutta siinä käytetty kieli on silti hyödyllistä. - This series is funny, but the language used in it is still useful.

It refers back to tämä sarja (“this series/show”). So siinä = in that (series).
Grammatically, siinä is the inessive singular of the pronoun se:
- se → siinä = in it / in that
It tells you where the language is used: “the language used in it (in that series)”.

You could also say tässä sarjassa (“in this series”), but siinä is very natural when you’ve just mentioned the thing (here, the series) and are now talking about something inside it.

How is the phrase siinä käytetty kieli built, and what is its literal meaning?

Siinä käytetty kieli breaks down as:

siinä – “in it / there” (inessive of se)
käytetty – past passive participle of käyttää (“to use”), meaning “used”
kieli – “language”

So literally: “the in-it used language” → more naturally in English:
“the language used in it”.

Grammatically:

käytetty is an attributive participle modifying kieli.
The whole thing works like an adjective + noun:
- hyvä kieli – good language
- siinä käytetty kieli – (the) language used in it

Finnish often prefers this participle structure instead of a relative clause.
Compare:

siinä käytetty kieli
kieli, jota siinä käytetään = “the language that is used in it”

Both are correct, but the participle version is shorter and very common.

Why is it käytetty and not käytetään?

Käytetty is used because the verb is acting like an adjective modifying kieli, not as the main verb of a clause.

käytetään is a finite verb form (present passive: “is used”).
To attach this idea directly to a noun, Finnish uses a participle, not a finite verb.

So:

kieli, jota siinä käytetään – a full relative clause (“the language that is used there”)
siinä käytetty kieli – participle phrase (“the language used there”)

Käytetty is:

past passive participle of käyttää
agrees in number and case with kieli (here nominative singular: käytetty kieli)

If kieli were in another case, the participle would follow:

siinä käytettyä kieltä – “(some) language used in it” (partitive)
siinä käytetystä kielestä – “about the language used in it” (elative)

Why is hyödyllistä in the partitive when kieli is in the nominative?

In Finnish, the predicate adjective after olla (to be) can be either nominative or partitive.
Here we have:

kieli – nominative subject
hyödyllistä – partitive singular predicate adjective

This is called a partitive predicative. It is used when:

The subject is treated as a mass / uncountable concept, or
You are talking about the quality in general, not as a specific, countable item.

Examples:

Maito on hyvää. – Milk is good. (milk as a substance)
Vesi on kylmää. – The water is cold.

Here kieli is understood more like “language as stuff / type of language”, not as “one specific language as an object”. So:

siinä käytetty kieli on silti hyödyllistä
≈ “The kind of language used in it is still useful (to learn).”

So the mismatch in case is normal: subject nominative, predicative partitive is a regular pattern with uncountable / qualitative meanings.

Can I say kieli on silti hyödyllinen instead? What is the difference?

Yes, siinä käytetty kieli on silti hyödyllinen is also grammatically correct.
The difference is in nuance:

hyödyllistä (partitive)
- treats kieli as a type / quality / mass
- “The sort of language used there is useful (as material / for learning).”
hyödyllinen (nominative)
- treats kieli more as a concrete entity / tool
- “The language used there is (a) useful language.”

Compare with named languages:

Ruotsi on hyödyllinen kieli. – Swedish is a useful language.
(You basically never say ruotsi on hyödyllistä in this sense.)

In your sentence, many Finns would actually prefer the partitive hyödyllistä, because we are evaluating the kind of language usage in the show (vocabulary, expressions, style) as useful material, not just labelling “the language” as a tool.

Both versions are understandable and correct; the partitive just sounds more natural for “useful kind of language” here.

Why is it Tämä sarja on hauska and not Tämä sarja on hauskaa?

With concrete, countable things like a specific series, movie, or book, the default is a nominative predicative:

Tämä sarja on hauska. – This series is funny.
Tuo elokuva oli tylsä. – That movie was boring.

Here sarja is a specific item, so we treat it as a countable object, and the predicate adjective agrees in number/gender only in a very simple way: nominative singular hauska.

The partitive hauskaa is used in different contexts, for example:

Se oli hauskaa. – That was fun.
- Here se often refers to an activity / event / experience (more like “it (the experience) was fun”), and Finnish tends to use partitive predicatives for that kind of “stuff-like” subject.

But saying Tämä sarja on hauskaa about a TV series is not normal standard Finnish; it sounds wrong or at least very odd. So in this sentence, hauska (nominative) is the correct choice.

What does silti mean here, and how is it different from kuitenkin?

Silti means “still / nevertheless / all the same”. It marks a contrast with what was said before:

The series is funny, but still the language used in it is useful.

Silti and kuitenkin are very close in meaning:

silti – “still, nevertheless, even so”
kuitenkin – “however, nevertheless, anyway”

In this sentence you could say either:

…kieli on silti hyödyllistä.
…kieli on kuitenkin hyödyllistä.

Differences:

kuitenkin is slightly more neutral / common in written language.
silti often feels a bit more direct and contrastive in speech.

But in everyday use, they overlap a lot, and both are fine here.

Where can silti go in the sentence? Is the word order flexible?

Yes, silti is somewhat flexible, but some positions are more natural than others.

The neutral, most common placement is:

…kieli on silti hyödyllistä.
(subject – verb – silti – predicate)

Other possible orders:

…silti siinä käytetty kieli on hyödyllistä.
- Emphasises the contrast: “Still, the language used in it is useful.”
- silti at the beginning of the clause is very common when you want to stress “nevertheless”.
…kieli silti on hyödyllistä.
- Possible, but more marked; emphasises that it really is useful, despite expectations.

Less natural:

*…kieli on hyödyllistä silti. – This feels awkward or poetic; silti is rarely placed at the very end in neutral prose.

So, safe patterns to remember:

silti right after the verb: on silti hyödyllistä
or at the start of the clause for emphasis: Silti kieli on hyödyllistä

What is the function of mutta here, and could I use vaikka instead?

Mutta is a coordinating conjunction meaning “but”. It links two independent clauses:

Tämä sarja on hauska – This series is funny
siinä käytetty kieli on silti hyödyllistä – the language used in it is still useful

So the structure is: [Clause 1], mutta [Clause 2].

Vaikka means “although / even though” and is a subordinating conjunction. You can’t simply replace mutta with vaikka in the same structure. You’d have to change the word order:

Vaikka tämä sarja on hauska, siinä käytetty kieli on silti hyödyllistä.
= “Although this series is funny, the language used in it is still useful.”

So:

mutta – joins two equal clauses, like English but
vaikka – introduces a subordinate clause, like English although

Why is there a comma before mutta in Finnish?

In standard Finnish punctuation, you normally put a comma before coordinating conjunctions like mutta, ja, tai when they connect two full clauses (each with its own subject and verb).

Here we have:

Tämä sarja on hauska – subject: Tämä sarja, verb: on
siinä käytetty kieli on silti hyödyllistä – subject: (siinä käytetty) kieli, verb: on

Because both parts are independent clauses, you write:

Tämä sarja on hauska, mutta siinä käytetty kieli on silti hyödyllistä.

Leaving out the comma here is considered incorrect in standard written Finnish.