Learn Hardcore Finnish: Minusta tämä vastaus on rehellinen.

Minusta is the elative case (-sta/-stä) of minä (I). Literally it means from me, but in expressions of opinion it is idiomatic and means in my opinion / I think.

So:

minä = I (nominative)
minusta = from me → in this usage: I think / in my opinion

This is a very common pattern in Finnish: using a locative case form of a person pronoun to express mental states or opinions:

Minusta tämä on hyvä. – I think this is good.
Sinusta tämä on tylsää. – You think this is boring.

In your sentence, minusta does not mean physical movement but introduces your opinion.

Is minusta the subject of the sentence?

No. The subject is tämä vastaus (this answer).

Breakdown:

Minusta – adverbial phrase expressing whose opinion it is (in my opinion).
tämä vastaus – subject (this answer).
on – verb to be, 3rd person singular, agreeing with tämä vastaus.
rehellinen – predicate adjective describing tämä vastaus.

So structurally, the sentence is like:

In my opinion, this answer is honest.

Minusta is not the grammatical subject; it’s more like an adverbial of viewpoint.

Why is it minusta and not minun mielestäni? Are minusta and minun mielestäni the same?

They are very close in meaning, but not identical in form.

Minusta tämä vastaus on rehellinen.
Minun mielestäni tämä vastaus on rehellinen.

Both mean essentially: In my opinion, this answer is honest.

Differences:

Minusta is shorter and more colloquial-sounding, very common in everyday speech.
Minun mielestäni is more explicit and a bit more formal or careful, because it literally means in my mind’s view:
- mieli = mind, opinion
- mielestäni = from my mind → in my opinion
- minun mielestäni = in my (own) opinion

In practice, you can usually use them interchangeably. Minusta is perfectly standard, not slang.

Can I drop minun and just say Mielestäni tämä vastaus on rehellinen?

Yes. Mielestäni tämä vastaus on rehellinen is completely correct and very common.

mielestäni already contains the -ni ending meaning my/my opinion, so minun is not required.
Mielestäni = in my opinion.

So these three are all natural:

Minusta tämä vastaus on rehellinen.
Mielestäni tämä vastaus on rehellinen.
Minun mielestäni tämä vastaus on rehellinen. (a bit more emphatic/explicit)

How is Minusta tämä vastaus on rehellinen different from Minä luulen, että tämä vastaus on rehellinen?

Both express a personal judgment, but with slightly different nuances.

Minusta tämä vastaus on rehellinen.
- Very neutral way to state an opinion.
- Often sounds like a more stable or evaluative judgment: To me / in my view, this answer is honest.
Minä luulen, että tämä vastaus on rehellinen.
- Literally: I think / I believe that this answer is honest.
- Emphasizes the mental process of believing or supposing.
- Can sound slightly more tentative or about your personal belief at that moment.

In everyday conversation, minusta… is the more typical way to give an opinion on qualities (good, bad, honest, boring, etc.). Minä luulen, että… is fine too, but often used for beliefs about facts (I think he’s at home, I think it will rain).

Why is rehellinen in this form and not rehellistä?

Rehellinen is a predicate adjective describing the subject tämä vastaus. In Finnish, predicate adjectives usually agree with the subject in case and number when the statement is about a definite, complete thing.

Here:

tämä vastaus – nominative singular subject
rehellinen – nominative singular adjective agreeing with it

So you get:

Tämä vastaus on rehellinen. – This answer is honest.

You would use rehellistä (partitive) in other structures, for example:

Tämä vastaus on rehellistä puhetta. – This answer is honest talk.
(Here rehellistä agrees with puhetta, which is in the partitive.)
Or in some incomplete / “some of” type statements you might see predicate partitive, but that’s a more advanced topic.

In your sentence, because you’re simply saying what the whole, specific answer is like, rehellinen (nominative) is the expected form.

Can I change the word order? For example, can I say Tämä vastaus on minusta rehellinen?

Yes, Finnish word order is quite flexible, and both are correct:

Minusta tämä vastaus on rehellinen.
Tämä vastaus on minusta rehellinen.

They mean the same thing, but the focus shifts slightly:

Starting with Minusta foregrounds whose opinion we’re talking about: As for me, this answer is honest.
Starting with Tämä vastaus foregrounds the answer itself: This answer is honest, in my opinion.

You can also sometimes put minusta at the end for a more “afterthought” feel:

Tämä vastaus on rehellinen, minusta.

All are grammatically fine; the choice is about emphasis and style.

How would this sentence look in casual spoken Finnish?

A very natural colloquial version would be:

Musta tää vastaus on rehellinen.

Changes:

minusta → musta (very common spoken contraction)
tämä → tää (spoken form)

This is how many Finns would actually say it in everyday speech, though in writing or formal contexts you would stick to Minusta tämä vastaus on rehellinen.

Does minusta only mean in my opinion, or can it also mean from me in a literal sense?

It can mean both, depending on context.

Opinion:
- Minusta tämä vastaus on rehellinen.
  → In my opinion, this answer is honest.
Literal from me:
- Ota kirja minusta vasemmalle. – Take the book to my left. (here minusta = from me / relative to me)
- Se ei ollut minusta kiinni. – It didn’t depend on me. (still somewhat abstract, but not exactly opinion)

Usually, context and verb choice make it clear whether it’s an opinion marker or a more literal/other usage. In your sentence, with tämä vastaus on rehellinen, it is clearly the opinion use.

Why is the verb on in 3rd person singular? Shouldn’t it agree with minä?

The verb on agrees with the subject tämä vastaus, not with minusta.

tämä vastaus – 3rd person singular subject
on – 3rd person singular form of olla (to be)

If minä were the subject, you would have:

Minä olen rehellinen. – I am honest.

But in your sentence, you are not saying I am honest; you are saying that this answer is honest in your opinion. That’s why:

Minusta tämä vastaus on rehellinen.
(literally: From me / to me, this answer is honest.)