Se on kirja, jota luen nyt.

It can feel like either, depending on context. In this sentence, se refers to some specific thing and can be understood as it or that in English.

Finnish se is often used for things that are already known from the situation or have just been mentioned. English usually has to choose more clearly between it and that, but Finnish is often less strict here.

Finnish does not have articles. So kirja can mean a book or the book depending on context.

The sentence becomes specific because of the relative clause jota luen nyt, which tells you which book is meant.

Because kirja is a predicate noun after on.

In a sentence like Se on kirja, Finnish normally uses the nominative singular for the noun that identifies or classifies something. So kirja is the expected form here.

jota is a relative pronoun. It links kirja to the clause luen nyt.

In English, this is the kind of word that often corresponds to that or which, as in the book that I am reading now.

Because inside the relative clause, the book is the object of luen.

The basic relative pronoun is joka, but it changes form according to its role in the clause. Here it is not the subject; it is the object. Since the action is ongoing, Finnish uses the partitive object, so the correct form is jota.

So:

• joka = nominative, often used for the subject
• jota = partitive, used here because the book is the object of an ongoing action

Yes. jonka can be used when the object is a total object rather than a partitive object.

A very useful contrast is:

• kirja, jota luen nyt = the book I am reading now
• kirja, jonka luen huomenna = the book I will read tomorrow

In the first one, the reading is in progress, so Finnish uses the partitive jota. In the second, the book is seen more as a whole completed object, so jonka is possible.

Finnish does not have a separate verb form exactly like the English present continuous.

The ordinary present tense luen can mean both:

• I read
• I am reading

The word nyt and the context make it clear that the action is happening now, so English naturally uses am reading.

The dictionary form is lukea, meaning to read.

luen is the 1st person singular present form, so it means I read or I am reading.

This verb has a common stem change:

• lukea = dictionary form
• luen = I read
• luet = you read
• lukee = he/she/it reads

So the form is not built by simply removing the ending from the dictionary form. The stem changes in a regular Finnish verb pattern.

Because Finnish often leaves out subject pronouns when the verb ending already shows the person clearly.

luen already tells you that the subject is I, so minä is unnecessary unless you want extra emphasis.

Both are possible:

• luen = I read / I am reading
• minä luen = I read / I am reading, with more emphasis on I

That is a natural word order in Finnish.

Finnish word order is more flexible than English word order, and the end position is a normal place for an adverb like nyt. It gives a neutral meaning here.

You could also say Se on kirja, jota nyt luen, but that puts slightly more focus on now.

Because Finnish normally separates a relative clause with a comma.

So in Se on kirja, jota luen nyt, the part jota luen nyt is a relative clause modifying kirja, and the comma is standard punctuation.

Because the antecedent here is the noun kirja.

When a relative pronoun refers back to a noun like kirja, standard Finnish normally uses joka and its case forms, so here jota.

mikä is used in other situations, for example when referring to a whole idea or clause rather than directly to a noun. In this sentence, jota is the normal choice.