Learn Hardcore Finnish: Minusta metro on kätevä iltaisin. - In my opinion the metro is convenient in the evenings.

Literally, minusta is “from me” (elative case of minä, “I”). Finnish often expresses opinions with an elative pronoun + a linking verb: Minusta X on Y = “In my opinion, X is Y.” You can do the same with other persons: sinusta, hänestä, meistä, teistä, heistä.

Can I say Minun mielestäni or Mielestäni instead of Minusta?

Yes.

Mielestäni metro on kätevä iltaisin. (quite formal/neutral written style)
Minun mielestäni metro on kätevä iltaisin. (explicit “my opinion”)
Minun mielestä metro on kätevä iltaisin. (colloquial; no possessive suffix) All mean the same here. Minusta is compact and completely natural too.

Where can minusta go in the sentence, and does word order change the meaning?

All of these are grammatical; word order only tweaks emphasis:

Minusta metro on kätevä iltaisin. (sets your opinion as the frame)
Metro on minusta kätevä iltaisin. (adds “in my view” more lightly)
Iltaisin metro on (minusta) kätevä. (emphasizes “in the evenings”)
Metro on kätevä iltaisin, minusta. (afterthought; often with a comma in writing)

Why is there no article before metro?

Finnish has no articles (no “a/the”). Bare metro can be specific or generic from context. If you need to force specificity, you use pronouns/demonstratives (e.g., se metro “that/the metro” in context), not articles.

Why isn’t metro inflected here?

As the subject, metro is in the nominative (dictionary) form. Other cases appear when required by meaning:

metron (genitive: “of the metro”)
metroa (partitive)
metrossa (in the metro), metrosta (from the metro), metroon (into the metro), etc. The sentence doesn’t call for any of those.

Why is it kätevä and not kätevää/kätevät/käteviä?

With a singular subject (metro), the predicative adjective is nominative singular: kätevä.

Plural subject: you’ll see either nominative plural (Metrot ovat kätevät) or partitive plural (Metrot ovat käteviä). Both occur; the partitive plural is common when speaking generally.
Kätevää (partitive singular) is used with abstract/mass-like subjects: Metrolla matkustaminen on kätevää (“Travelling by metro is convenient”). But Metro on kätevää is odd.

What’s the nuance of kätevä compared to käytännöllinen or helppo?

kätevä = handy, convenient for practical use (good to have, saves effort/time).
käytännöllinen = practical, well-suited in practice/design (vs. theoretical or decorative).
helppo = easy (low effort/skill required). In this sentence, kätevä is the idiomatic choice.

What exactly does iltaisin mean compared to illalla or iltoina?

iltaisin = habitually/typically “in the evenings” (distributive time adverb).
illalla = “in the evening” (a specific evening, e.g., tonight).
iltoina = “on (certain) evenings” (plural essive; often with quantifiers: joinakin iltoina “on some evenings”). Other common -sin forms: aamuisin (in the mornings), päivisin (in the daytime), öisin (at night).

Can iltaisin be placed elsewhere?

Yes:

Iltaisin metro on kätevä. (foregrounds time)
Minusta iltaisin metro on kätevä.
Metro on iltaisin kätevä. All are fine; position controls focus/emphasis, not truth conditions.

Is on required? How do I negate or ask a question?

Yes, the copula on is required.

Negation: Minusta metro ei ole kätevä iltaisin.
Yes/no question (about your opinion): Onko metro iltaisin kätevä sinun mielestäsi?
Short answers: On, minusta on. / Ei ole, minusta ei ole.

Does minusta always express an opinion?

No. Minusta is “from me,” so it appears in other meanings too:

Physical source: Ota se minusta pois. (“Take it off me.”)
Feeling/perception: Minusta tuntuu, että… (“I feel that…”)
Dependence: Se riippuu minusta. (“It depends on me.”) Context disambiguates it from the opinion use.

How would I say this more colloquially?

Mun mielestä metro on kätevä iltaisin. (very common speech)
In colloquial spelling, minusta often becomes musta: Musta metro on kätevä iltaisin. Note: musta also means “black,” so rely on context in writing.

Can I use the adverb kätevästi instead of kätevä?

Use kätevästi to modify a verb, not to be the predicate of metro:

Metro kulkee kätevästi iltaisin. (“The metro runs conveniently in the evenings.”)
Your original uses an adjective with olla: Metro on kätevä iltaisin.

How do I make comparisons or superlatives with kätevä?

Comparative: kätevämpi — Minusta metro on kätevämpi kuin bussi iltaisin.
Superlative: kätevin — Minusta metro on iltaisin kätevin vaihtoehto. Adverbially: kätevämmin (“more conveniently”).

Could I start with time instead: “In the evenings, I think the metro is convenient”?

Yes: Iltaisin minusta metro on kätevä. This puts the time frame first, then your stance (minusta), then the statement. It’s a natural way to highlight “in the evenings.”