Learn Hardcore Croatian: Večeras učim ovu lekciju. - Tonight I am studying this lesson.

učim by itself already means I study and I am studying.
The -m ending shows 1st person singular (I), and the present tense is built directly on the verb stem.

So učim covers both English meanings: I study and I am studying.

Can Večeras učim ovu lekciju. mean a future plan, like I’m going to study this lesson tonight?

Yes.

Even though učim is a present tense form, adding večeras (this evening) makes it clear we are talking about a plan for later today. In Croatian:

Present tense + future time adverb (sutra, večeras, sljedeći tjedan, etc.) often expresses a scheduled or planned future.

So Večeras učim ovu lekciju. can be understood as:

I’m studying this lesson tonight
or
I’m going to study this lesson tonight.

What is the difference between učiti and studirati?

Both can translate as to study, but they’re used differently:

učiti = to learn / to study (specific material, a lesson, words, for an exam, etc.)
- Učim ovu lekciju. = I’m learning / studying this lesson.
- Učim hrvatski. = I’m learning Croatian.
studirati = to study (be a student of a subject at a university or college)
- Studiram pravo. = I study law / I’m a law student.
- Studira u Zagrebu. = He/She studies in Zagreb.

In Večeras učim ovu lekciju., učim is correct, because you are working on a specific lesson.

Why is it ovu lekciju and not ova lekcija?

It’s because of case. The verb učiti takes a direct object in the accusative case (the thing being learned).

Dictionary/base forms:
- ova lekcija = this lesson (nominative, used for the subject)
As a direct object (accusative, feminine singular):
- ovu lekciju = this lesson (object of the verb)

So:

Ova lekcija je teška. = This lesson is hard. (subject, nominative)
Učim ovu lekciju. = I am studying this lesson. (object, accusative)

What are the different forms of ova (this), and why is ovu used here?

Ovaj / ova / ovo is the demonstrative meaning this:

Masculine singular: ovaj (nominative) → accusative: ovaj / ovog (depending on animacy)
Feminine singular: ova (nominative) → accusative: ovu
Neuter singular: ovo (nominative and accusative)

lekcija is feminine, singular, and it’s an object, so we need:

feminine + singular + accusative → ovu

Hence: ovu lekciju.

Why is lekciju spelled like that? What is the base form?

The base (dictionary) form is lekcija (feminine noun, nominative singular).

For feminine nouns like lekcija, the accusative singular often ends in -u:

Nominative: lekcija (subject form)
Accusative: lekciju (object form)

Because lekciju is the direct object of učim, it must be in the accusative.

Why is there no word for the or a before lekciju?

Croatian does not have articles like English a / an / the.

lekcija can mean a lesson or the lesson, depending on context.
ovu lekciju is already specific because ovu means this, so it functions a bit like this (particular) lesson.

Definiteness and indefiniteness (whether it is a or the) are understood from context, not from separate words.

Can I say Ja večeras učim ovu lekciju.? Is ja required?

You can say it, but ja is not required.

učim already shows 1st person singular (I) from the -m ending.
Subject pronouns (ja, ti, on, etc.) are often dropped in Croatian unless you want to:
- Emphasize the subject, or
- Clarify who you are talking about.

So:

Večeras učim ovu lekciju. = normal, neutral
Ja večeras učim ovu lekciju. = I am studying this lesson tonight (maybe in contrast to someone else)

Can I change the word order, for example: Učim ovu lekciju večeras.? Does the meaning change?

Yes, you can change the word order; the basic meaning stays the same.

Possible orders:

Večeras učim ovu lekciju. (very natural; As for tonight, I’m studying this lesson.)
Učim ovu lekciju večeras. (slightly more neutral: I’m studying this lesson tonight.)
Ovu lekciju učim večeras. (emphasis on this lesson rather than another one)

Croatian word order is relatively flexible; changing it usually affects emphasis or what is considered “old” vs “new” information, not the core meaning.

What is the difference between večeras and noćas?

Both can be translated as tonight, but they focus on different parts of the night:

večeras = this evening (roughly from late afternoon/early evening up to around 10–11 pm)
noćas = tonight, during the night (usually after you’d call it evening, e.g. late night or in your sleep hours)

So:

Večeras učim ovu lekciju. = I’ll be studying in the evening.
Noćas učim ovu lekciju. = I’ll be studying during the (late) night.

What is the verb učiti like in the present tense? How do I conjugate it?

učiti is a regular -iti verb. Present tense:

ja učim = I learn / study
ti učiš = you learn / study (singular, informal)
on / ona / ono uči = he / she / it learns / studies
mi učimo = we learn / study
vi učite = you learn / study (plural or formal)
oni / one / ona uče = they learn / study

In the sentence, učim is 1st person singular: I learn / I study.

Is there a perfective partner of učiti, and would that change the meaning of the sentence?

Yes, the common perfective partner is naučiti.

učiti (imperfective) = to be in the process of learning / studying
naučiti (perfective) = to learn something to completion, to have mastered it

Compare:

Večeras učim ovu lekciju.
= Tonight I will be studying / working on this lesson (process, not necessarily finished).
Večeras ću naučiti ovu lekciju.
= Tonight I will learn / will have learned this lesson (aiming at completion or mastery).