Learn Hardcore Finnish: Opin parhaiten uusia sanoja lukemalla ääneen joka päivä. - I learn new words best by reading aloud every day.

opin is the 1st person singular form of oppia (to learn).
The -n ending tells you the subject is “I” (minä).
You could say Minä opin parhaiten…, but it usually sounds unnecessary unless you want to emphasize I (as opposed to someone else).

What exactly does opin mean, and how do I know if it’s present or past tense?

Opin is the 1st person singular of oppia and can be:

present: (minä) opin = I learn / I am learning
past: (minä) opin = I learned

In the 1st person singular, the present and past forms look the same; you tell them apart by context.

In a general statement like this, it’s usually understood as present: I (generally) learn new words best….
In a narrative context, it could be past: I learned new words best by reading aloud every day (e.g. describing your study method in the past).

Why is it parhaiten and not paras or parhaasti?

Finnish distinguishes between adjectives (modifying nouns) and adverbs (modifying verbs).

hyvä = good (adjective)
parempi = better (adjective)
paras = best (adjective, “the best” thing)
hyvin = well (adverb)
paremmin = better (adverb)
parhaiten = best (adverb)

In the sentence, parhaiten is modifying the verb opin – it answers how / in what way do I learn?:

Opin parhaiten = I learn best (manner: best).

Paras would be used before a noun:

paras kirja = the best book.

There is no parhaasti; the correct superlative adverb is parhaiten.

Why is it uusia sanoja and not uudet sanat?

Both uusia sanoja and uudet sanat are grammatically possible, but they are different in meaning.

uusia sanoja
- uusia = new (adjective in partitive plural)
- sanoja = words (noun in partitive plural)
- This expresses an indefinite quantity or ongoing/partial learning:
  - I learn (some) new words, new words in general, not a fixed set.
- This is the natural choice when talking about learning vocabulary in general.
uudet sanat
- uudet = new (adjective in nominative plural)
- sanat = words (noun in nominative plural or total object form)
- Would suggest a specific, complete set:
  - the new words (that we talked about / from this list / from this chapter).

In general, oppia + partitive (uusia sanoja) suggests learning some amount of something (process, not limited), which fits the idea of “learning new words” as an ongoing activity.

Why are both uusia and sanoja in the partitive plural? Why not just the noun?

In Finnish, adjectives usually agree in case and number with the noun they modify.

Noun: sanoja (partitive plural of sana)
Adjective: uusia (partitive plural of uusi)

Because sanoja is in the partitive plural, uusia must also be in partitive plural:

uusia sanoja = new words (some amount of new words)

You normally don’t mix cases like uusia sanat or uudet sanoja; that would be wrong here.

What form is lukemalla, and what does it literally mean?

Lukemalla is the 3rd infinitive in the adessive case of the verb lukea (to read).

Verb stem: luke-
3rd infinitive: lukema
Adessive ending: -lla / -llä
→ lukemalla

This form often expresses means or manner and corresponds to English “by doing”:

lukemalla = by reading / through reading

So Opin parhaiten uusia sanoja lukemalla… = I learn new words best *by reading…
Finnish doesn’t need a separate word for *“by”; the meaning is built into the -malla/-mällä ending.

Why do we say lukemalla ääneen and not something like lukemalla ääntä or lukemalla kovaa?

The expression lukea ääneen is an idiomatic phrase in Finnish meaning “to read aloud”.

ääni = voice, sound
ääneen is the illative form of ääni (“into the voice / into sound”), but in modern language it basically functions as an adverb meaning aloud, out loud.

So:

lukea ääneen = to read aloud
lukemalla ääneen = by reading aloud

Alternatives like:

lukemalla ääntä – would literally be “reading sound” (ungrammatical/odd here)
lukemalla kovaa – would be understood as “by reading loudly” (focusing on volume, not the idea of reading out loud vs silently)

For “aloud vs silently”, ääneen is the standard choice.

Why is it joka päivä and not jokat päivää or joka päivää?

Joka (“each, every”) has special behavior:

It always takes the singular nominative form of the noun after it.
It does not agree in case or number with the main sentence.

So:

joka päivä = every day (literally: each day)
- joka = each/every
- päivä = day (singular nominative)

You do not inflect päivä into partitive (päivää) or plural (päivät) after joka in this kind of time expression.

Forms like jokat päivää or joka päivää are incorrect here.

Can the word order change? For example, can I say Joka päivä opin parhaiten uusia sanoja lukemalla ääneen?

Yes, Finnish allows fairly flexible word order, and your example is grammatical.

Some natural variants:

Opin parhaiten uusia sanoja lukemalla ääneen joka päivä. (original)
Opin parhaiten uusia sanoja joka päivä lukemalla ääneen.
Joka päivä opin parhaiten uusia sanoja lukemalla ääneen.
Uusia sanoja opin parhaiten lukemalla ääneen joka päivä.

The basic meaning stays the same. Word order mostly affects:

emphasis (what’s highlighted)
- Starting with Joka päivä emphasizes the daily routine.
- Starting with Uusia sanoja emphasizes what you learn.
flow / style, but all of these would be understood the same way in context.

Could I use opiskella instead of oppia, as in Opiskelen parhaiten uusia sanoja lukemalla ääneen joka päivä?

You can say that, but oppia and opiskella are not exact synonyms.

oppia = to learn (gain knowledge/skill, result-focused)
- Opin parhaiten uusia sanoja… = I learn new words best…
opiskella = to study (do the activity of studying, process-focused)
- Opiskelen parhaiten uusia sanoja… = I study new words best…
  (emphasis on the studying activity, not explicitly on the result of learning)

Both are understandable, but in this sentence oppia is more natural because you’re talking about the effectiveness of your method (how you learn best), not just that you study.