Learn Hardcore Finnish: Opiskelijana luen paljon.

In this sentence, Opiskelijana luen paljon means As a student, I read a lot.
Using plain opiskelija would be ungrammatical here; the structure specifically needs the essive to show the role or state in which the action happens.

What exactly does opiskelijana express here – time, role, or condition?

It mainly expresses role or state, with a bit of a time/phase of life nuance.

Opiskelijana luen paljon can be understood as:

In my role as a student, I read a lot.
During the period when I am a student, I read a lot.

So it’s both:

a role: you are functioning as a student
a stage/period: the time in your life when you are a student

This same essive pattern is used in:

Lapsena luin paljon. = As a child / When I was a child, I read a lot.
Opettajana työskentelen Turussa. = As a teacher, I work in Turku.

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

Finnish normally drops subject pronouns, because the verb ending already shows the person.

luen = I read
- -n ending = 1st person singular

So:

Luen paljon. = I read a lot. (without needing minä)
Opiskelijana luen paljon. = As a student, I read a lot.

You can add minä for emphasis or contrast:

Opiskelijana minä luen paljon, mutta kaverini ei lue.
= As a student, I read a lot, but my friend doesn’t.

But the default/neutral way is to leave minä out.

What part of speech is paljon, and what does it literally mean?

Paljon is an adverb meaning a lot, much, or many (in total), depending on context.

Historically it comes from the partitive form of an old word paljo, but in modern Finnish you mostly just treat paljon as an adverb. It typically modifies:

verbs:
- Luen paljon. = I read a lot.
- Syön paljon. = I eat a lot.
or quantities with objects:
- Luen paljon kirjoja. = I read a lot of books.

Is there something “missing” after paljon, like books?

It’s implied but not required.

Luen paljon.
Literally: I read a lot (in general).
The object (for example books, articles, texts) is understood from context or left general.

If you want to be explicit, you can say:

Luen paljon kirjoja. = I read a lot of books.
Luen paljon artikkeleita. = I read a lot of articles.

But Finnish allows the more general Luen paljon without specifying what you read.

Why is it paljon here but paljoa in some other sentences?

Paljon and paljoa are closely related, but paljoa is mainly used in negative or restrictive contexts.

Positive / neutral:
- Luen paljon. = I read a lot.
- Täällä on paljon ihmisiä. = There are many people here.
Negative / limiting:
- En lue paljoa. = I don’t read much.
- En jaksa syödä paljoa. = I can’t manage to eat much.

In your sentence, the meaning is clearly positive (you do read a lot), so paljon is the normal choice.

Can I change the word order? For example, can I say Luen paljon opiskelijana?

You can, but it changes the feel of the sentence slightly and is less typical in this context.

Opiskelijana luen paljon.
- Neutral, natural order here.
- Emphasis on the situation as a student; it frames the whole sentence.
Luen paljon opiskelijana.
- Grammatically possible.
- Can sound a bit like you are contrasting this role with some other role:
  Luen paljon opiskelijana, mutta en lue paljon lomalla.
  = I read a lot as a student, but not much on holiday.

In general, starting with Opiskelijana is the most natural way to express this idea.

Do I need a comma after Opiskelijana?

No comma is needed here.

In Finnish, a simple adverbial (like opiskelijana, kesällä, koulussa) placed at the beginning of a clause does not take a comma before the main clause:

Opiskelijana luen paljon.
Kesällä matkustan paljon. = In summer I travel a lot.

You would use a comma if there were a full subordinate clause at the beginning:

Kun olin opiskelija, luin paljon. = When I was a student, I read a lot.

Does Opiskelijana luen paljon mean right now, in general, or in the future?

Finnish present tense often covers:

present ongoing actions
general habits (like English I read a lot)
near future in some contexts

Here, luen is best understood as habitual:

As a student, I read a lot (as a general fact/habit).

It does not specifically mean I am reading a lot at this exact moment (though context can sometimes allow that), but rather what you typically do as a student.

How is luen formed from lukea?

Lukea is the basic (dictionary) form of the verb to read.
It’s a type 1 Finnish verb (ending in -ea/-eä).

The present tense 1st person singular is formed by:

Take the stem: luke-
Add the personal ending -n

Result: luen = I read

Other present forms for comparison:

luet = you read (singular)
lukee = he/she reads
luemme = we read
luette = you read (plural)
lukevat = they read

Could I say Opiskelijana minä luen paljon instead? What changes?

Yes, that is grammatically correct. The meaning stays almost the same, but:

Opiskelijana luen paljon.
- Neutral; no special emphasis on I.
Opiskelijana minä luen paljon.
- Emphasizes minä = I.
- Implies contrast, for example:
  - Opiskelijana minä luen paljon, mutta ystäväni ei.
    = As a student, I read a lot, but my friend doesn’t.

So you mainly add minä when you want contrast or emphasis.

Does opiskelijana always refer to the subject of the verb luen?

In this sentence, yes: opiskelijana describes the subject (the person who reads).

By default, an essive-form like opiskelijana at the start of the clause:

attaches to the subject of the main verb
tells you in what role/state that subject is doing the verb’s action

So:

Opiskelijana luen paljon.
= I, as a student, read a lot.

If you wanted it to refer to someone else, you’d normally need a more complex structure, for example a separate clause.