Learn Hardcore Finnish: Tämä tehtävä on haastava, mutta opin paljon. - This task is challenging, but I learn a lot.

Tämä – this (demonstrative pronoun, nominative singular)
tehtävä – task, exercise (noun, nominative singular)
on – is (3rd person singular of olla, to be)
haastava – challenging (adjective, nominative singular)
mutta – but (coordinating conjunction)
opin – I learn / I learned (1st person singular of oppia, to learn)
paljon – a lot, much, many (quantifier/adverb)

So structurally it is “this task is challenging, but I learn a lot.”

Why is it tämä tehtävä and not just tämä?

Tämä on its own can mean “this (thing)”, but that is vague.

Tämä tehtävä = this task (specifically)
You are pointing to a particular exercise or task.

If you said only Tämä on haastava, it would mean “this is challenging”, but the listener would have to infer what “this” refers to from context (maybe a task, maybe something else). Adding tehtävä makes the subject explicit, just like English “this task” instead of just “this”.

Why are both tämä tehtävä and haastava in the basic form (no endings)? Why not tämän tehtävän or haastavaa?

This is a basic “X is Y” sentence:

Subject: Tämä tehtävä – in nominative (the basic dictionary form)
Predicative (what the subject is): haastava – also in nominative

In Finnish, when you say “X is Y” and you are just identifying or describing the whole thing, both sides usually appear in the nominative:

Tämä tehtävä on haastava. – This task is challenging.

You would use forms like tämän tehtävän if the noun had another grammatical role, for example:

Pidän tästä tehtävästä. – “I like this task.”
Teen tämän tehtävän. – “I will do this task.”

Here, because tämä tehtävä is simply the subject, it stays in the nominative, and haastava agrees with it in nominative too.

Is tehtävä a noun or some kind of verb form? It ends in -tävä.

It’s both, historically.

Grammatically here, tehtävä is a noun meaning task / exercise.
Morphologically, it comes from the verb tehdä (to do, to make).
tehtävä is originally the -tava/-tävä participle, something like “(something) to be done”.

This participle form has become a very common noun meaning a task, an assignment, an exercise. So in your sentence you can just think of tehtävä as an ordinary noun: “a task”.

How do I know the tense of opin? Does it mean “I learn” or “I learned”?

Opin can be either:

present: “I learn / I am learning”
past (preterite): “I learned”

The forms happen to be identical in 1st person singular.

You normally tell the tense from context:

With clear present context:
Tämä tehtävä on haastava, mutta opin paljon.
→ This task is challenging, but I (do) learn a lot / I learn a lot (from it).
With a past-time adverb:
Eilen tehtävä oli haastava, mutta opin paljon.
→ The task was challenging yesterday, but I learned a lot.

So both readings are grammatically possible; the time frame must come from the surrounding context or adverbs.

Why is there no minä before opin? How do we know it means “I”?

In Finnish, personal pronouns are often dropped because the verb ending already shows the person:

opin: the ending -n marks 1st person singular → I learn / I learned
opit: 2nd person singular → you learn / you learned
opimme: 1st person plural → we learn / we learned

So minä opin and opin usually mean exactly the same:

Minä opin paljon.
Opin paljon.

You only add the pronoun (like minä) when you want to emphasise it:

Minä opin paljon (vaikka muut eivät opi).
I learn a lot (even if others don’t).

What’s the difference between opin and opiskelen? Could I say “mutta opiskelen paljon” instead?

They are related but not the same:

oppia → opin = to learn (focus on the result, knowledge gained)
opiskella → opiskelen = to study (focus on the activity, the process)

In your sentence:

mutta opin paljon – but I learn a lot / but I (do) learn a lot from it.
Emphasis: you actually gain a lot of new knowledge or skills.

If you say:

mutta opiskelen paljon – but I study a lot.
Emphasis: you spend a lot of time/effort studying, not necessarily that you successfully learn.

Both are grammatically fine, but they say slightly different things.

What exactly does paljon do here? Is it an object or an adverb?

Paljon is a quantifier that works a bit like an adverb here:

With opin, it tells how much you learn → “I learn a lot.”

Historically, paljon is the partitive form of an old noun paljo, but in modern Finnish it usually behaves like an adverb of quantity.

You can expand it:

Opin paljon asioita. – I learn many things / a lot of things.
Here asioita (things) is the partitive plural object, and paljon quantifies it.

In your original sentence there is no explicit object, so paljon just modifies opin directly.

Could I change the word order, for example: “Opin paljon, mutta tämä tehtävä on haastava”?

Yes. Finnish word order is fairly flexible, and your alternative is perfectly natural:

Tämä tehtävä on haastava, mutta opin paljon.
→ Starts with the difficulty, then contrasts it with the learning.
Opin paljon, mutta tämä tehtävä on haastava.
→ Starts with the learning, then adds that the task is challenging anyway.

Both are correct. The main effect of changing word order is information focus: what you put first tends to sound more topical or emphasised.

What you generally cannot do in neutral style is break up the basic pair too oddly, e.g.:

?Mutta paljon opin. – This is possible only in very specific emphasis contexts; it sounds poetic or strongly focused, not neutral everyday speech.

Why is there a comma before mutta?

In Finnish, a comma is normally used before coordinating conjunctions like:

ja (and)
mutta (but)
tai (or)
vaan (but rather)

when they connect two independent clauses.

In your sentence, you actually have two full clauses:

Tämä tehtävä on haastava
(minä) opin paljon

Each has its own verb and (implicit or explicit) subject, so they are independent. That is why there is a comma:

… on haastava, mutta opin paljon.

Could I say “Tämä on haastava tehtävä” instead of “Tämä tehtävä on haastava”? Is there a difference?

Both are grammatical, but the focus is a bit different:

Tämä tehtävä on haastava.
- Subject = this task.
- You are describing an already identified task.
- Feels a bit more like you are talking about that particular exercise.
Tämä on haastava tehtävä.
- Subject = this (this thing here), and you identify it as a challenging task.
- Feels more like: “This (whatever it is) is a challenging task.”

In many contexts they are interchangeable, but:

If you are already in the middle of doing the exercise and talking about it as a task, Tämä tehtävä on haastava is very natural.
If you are introducing something as “a kind of task”, Tämä on haastava tehtävä might be more likely.

Could I add siitä and say “mutta opin siitä paljon”? What’s the difference?

Yes, that’s idiomatic and common:

mutta opin paljon – but I learn a lot (in general / from it, understood from context).
mutta opin siitä paljon – but I learn a lot *from it.*

Here:

siitä = “from it” (elative form of se, that/it).

Adding siitä makes the source of your learning explicit: you are clearly saying you learn from this task. Without siitä, that idea is usually still understood from context, but it’s not spelled out.

How should I pronounce this sentence?

Rough guide (using approximate English sounds):

Tämä – TÄ-mä
- ä like in cat (but a bit tenser), both times.
tehtävä – TEH-tä-vä
- eh like in bed,
- ht pronounced almost like a long h+t sequence,
- ä again like in cat.
on – like English on but shorter.
haastava – HAA-sta-va
- aa is a long a, held longer than in English ha-,
- st as in English st,
- stress on the first syllable HAA.
mutta – MUT-ta
- u like in put (British) or book,
- tt is a long t, hold the closure briefly before releasing.
opin – O-pin
- o like in not (British) or off,
- stress on O.
paljon – PAL-yon
- a as in father (but shorter),
- lj is like ly (soft l + y-sound), somewhat like in million,
- final -on like on but short.

Remember: in Finnish, stress is almost always on the first syllable of each word, and double consonants (like tt in mutta) are held longer than single ones.