Learn Hardcore Finnish: Opin paljon lukemalla keskustelufoorumilla muiden kysymyksiä ja vastauksia. - I learn a lot by reading other people’s questions and answers on the discussion forum.

Context usually tells you which one is meant. Without a time adverb (like eilen = yesterday), this type of sentence is often understood as something habitual or generally true, so you can safely translate it as I learn a lot by reading….

Why is there no minä (I) in the sentence?

Finnish usually leaves out subject pronouns because the person is clear from the verb ending.

opin already tells you the subject is I.
Adding minä opin paljon… is grammatically correct but sounds more emphatic, like I (as opposed to someone else) learn a lot…

So the neutral, natural version is without minä:

Opin paljon… = I learn a lot…

What exactly does lukemalla mean, and how is it formed?

Lukemalla means “by reading” or “through reading”.

It is the third infinitive in the adessive case, formed like this:

Verb: lukea (to read)
Stem: luke-
Third infinitive base: lukema-
Adessive ending: -lla

→ lukemalla

This -malla / -mällä form is used to express “by doing X”:

Opin paljon lukemalla = I learn a lot by reading.
Laihdutin juoksemalla = I lost weight by running.

So lukemalla directly answers how? or by what means? you learn.

Could I use lukeminen instead of lukemalla? What’s the difference?

You could say:

Opin paljon lukemisesta.

Here the structure changes:

lukemalla = by reading (instrument, method)
lukemisesta (elative case) = from reading (source)

Nuance:

Opin paljon lukemalla… – focuses on the activity as the means: I learn a lot by reading…
Opin paljon lukemisesta… – more like I learn a lot *from reading…* (treating reading as a “source” of knowledge).

Both are correct; lukemalla is more typical when you’re describing a method.

Why is it keskustelufoorumilla and not keskustelufoorumissa?

Both -lla and -ssa can translate as on/at/in in English, but they have different basic meanings:

-lla / -llä (adessive): on, at (surface or general location)
-ssa / -ssä (inessive): in, inside

Keskustelufoorumi is an online discussion forum, like a website. Finnish often uses -lla for:

websites: Facebookissa (actually -ssa, but many sites take -ssa or -lla by convention), Redditissä, YouTubessa, netissä, but also:
more physical “surfaces/places”: pöydällä (on the table), asemalla (at the station)

With foorumi, keskustelufoorumilla (on/at a discussion forum) is the natural choice; it feels like being on the platform rather than inside a physical space.

Keskustelufoorumissa would sound a bit odd for an online forum, as if it were a physical room.

What is keskustelufoorumilla made of? Why is it one word?

Keskustelufoorumilla is a compound word + case ending:

keskustelu = discussion
foorumi = forum
keskustelufoorumi = discussion forum
keskustelufoorumilla = on/at a discussion forum (adessive -lla)

In Finnish, when two nouns together form a single concept (like discussion forum, coffee cup, school bus), they are usually written as one word:

keskustelufoorumi = discussion forum
kahvikuppi = coffee cup
koulubussi = school bus

Then you simply add the case ending to the whole compound:
keskustelufoorumi + -lla → keskustelufoorumilla.

What does muiden mean, and what form is it?

Muiden means “of others” / “other people’s”.

It is:

from the adjective/pronoun muu = other, another
genitive plural form: muu → muiden

So muiden kysymyksiä ja vastauksia literally is:

kysymyksiä ja vastauksia = questions and answers
muiden = of others / belonging to others

→ other people’s questions and answers.

Why are kysymyksiä and vastauksia in the partitive plural?

Both kysymyksiä and vastauksia are partitive plural:

kysymys → kysymyksiä
vastaus → vastauksia

They are in the partitive because:

The amount is unspecified / incomplete – you’re reading some questions and answers, not all of them.
Finnish often uses the partitive plural for “some X, X in general”:
- Luen kirjoja. = I read books (some books, books in general).

So:

muiden kysymyksiä ja vastauksia ≈ some of other people’s questions and answers or simply other people’s questions and answers in natural English.

Could I say muiden kysymykset ja vastaukset instead? What would change?

Yes, grammatically you can say muiden kysymykset ja vastaukset, but the meaning shifts:

muiden kysymyksiä ja vastauksia (partitive plural)
→ some of other people’s questions and answers / questions and answers (in general) from others
muiden kysymykset ja vastaukset (nominative plural)
→ all the questions and answers of the others or the others’ questions and answers as a more complete, specific set.

In your sentence, we’re talking about reading various questions and answers in general, so the partitive plural is more natural.

Why is the order keskustelufoorumilla muiden kysymyksiä ja vastauksia? Could I move parts of it around?

Yes, Finnish word order is fairly flexible. For example, you could also say:

Opin paljon lukemalla muiden kysymyksiä ja vastauksia keskustelufoorumilla.

Both versions are correct. Differences:

keskustelufoorumilla muiden kysymyksiä ja vastauksia
→ Slight emphasis on where you are reading them (on a forum).
muiden kysymyksiä ja vastauksia keskustelufoorumilla
→ Slight emphasis on what you’re reading (other people’s questions and answers), and then specifying where.

The basic meaning is the same. The original order just flows naturally in Finnish and keeps keskustelufoorumilla lukemalla close together in the mind (by reading on a discussion forum).

Why do we use ja between kysymyksiä and vastauksia, and do both words share the same case?

Yes, both words share the same case:

kysymyksiä = partitive plural
vastauksia = partitive plural

When you have A ja B (A and B) in Finnish, you normally put both in the same case, even if only the first one “shows” the reason clearly.

So here:

muiden [kysymyksiä ja vastauksia]
→ One “big” phrase: other people’s [questions and answers]
→ Both kysymyksiä and vastauksia are in partitive plural because they function the same way in the sentence (they are both things being read).

What does paljon add here? Could I just say Opin lukemalla…?

Paljon means “a lot, much” and tells how much you learn.

Opin lukemalla keskustelufoorumilla muiden kysymyksiä ja vastauksia.
→ I learn by reading… (neutral, no quantity)
Opin paljon lukemalla keskustelufoorumilla muiden kysymyksiä ja vastauksia.
→ I learn a lot by reading… (adds intensity/amount)

You can omit paljon and the sentence is still grammatically correct, but you lose the idea that you learn a lot from this activity.

Could this sentence also mean “I learned a lot…” (past), not just “I learn a lot…” (present)?

Yes. As mentioned earlier, opin can be both:

present: I learn / I am learning
imperfect (past): I learned

So the sentence could be:

I learn a lot by reading other people’s questions and answers on a discussion forum.
or
I learned a lot by reading other people’s questions and answers on a discussion forum.

In real use, context decides. If you’re telling a story about something you did, it will be understood as past; if you’re describing your general learning habits, it will be understood as present.