Plan je da večeras učimo hrvatski.

Croatian has no articles, so there is no separate word for the or a/an.

So plan can mean:

• the plan
• a plan

The exact meaning comes from context. In this sentence, English naturally uses the plan, so Plan je... is understood as The plan is...

Here da introduces a subordinate clause: da večeras učimo hrvatski.

A very literal breakdown is:

• Plan je = The plan is
• da večeras učimo hrvatski = that tonight we study Croatian

In natural English, you would usually say The plan is to study Croatian tonight rather than The plan is that...

So in this sentence, da is functioning a bit like that, but the whole structure is best translated naturally, not word-for-word.

This is very common in Croatian.

After da, Croatian often uses the present tense to talk about something that is planned, expected, wanted, or intended in the future.

So:

• da učimo literally looks like that we study / are studying
• but here it means that we will study or more naturally to study

The time word večeras makes the future meaning clear.

Učimo is the 1st person plural present tense of učiti.

It means:

• we study
• we are studying
• in this kind of sentence, we will study / we are going to study

The ending -mo tells you the subject is we, so Croatian does not need a separate pronoun.

Because Croatian is a language where the verb ending usually already shows the subject.

• učim = I study
• učiš = you study
• učimo = we study

So mi is normally omitted unless you want to add emphasis or contrast.

For example:

• Plan je da večeras učimo hrvatski. = neutral
• Plan je da mi večeras učimo hrvatski. = emphasizes we

Večeras means tonight or this evening.

It is an adverb of time, so it does not change form here.

A few useful comparisons:

• večer = evening
• večeras = tonight / this evening

So da večeras učimo hrvatski means that we study Croatian tonight.

In Croatian, names of languages and most adjectives of nationality are usually not capitalized.

So:

• hrvatski = Croatian
• engleski = English
• njemački = German

This is different from English spelling rules.

Both are possible.

• učimo hrvatski = we are studying Croatian
• učimo hrvatski jezik = we are studying the Croatian language

In everyday speech, Croatian often omits the noun jezik when the meaning is obvious. So hrvatski is standing in for hrvatski jezik.

This is very common with languages.

Because učiti takes a direct object, and that object is in the accusative case.

The full version would be:

• učimo hrvatski jezik

Here:

• jezik is masculine singular inanimate
• its accusative singular is the same as its nominative singular
• the adjective hrvatski agrees with it

So the shortened form učimo hrvatski keeps that same shape.

Yes. Croatian word order is fairly flexible.

This sentence could also appear as:

• Plan je da učimo hrvatski večeras.
• Večeras je plan da učimo hrvatski.

But the original:

• Plan je da večeras učimo hrvatski.

sounds natural and puts večeras in a clear, prominent position.

So the word order here is normal, but not the only possible one.

Because učiti focuses on the activity/process of studying or learning, while naučiti usually focuses on learning something successfully / mastering it.

So:

• učiti hrvatski = to study/learn Croatian
• naučiti hrvatski = to learn Croatian completely / to manage to learn it

If the plan is simply to spend the evening studying, učiti is the natural choice.

Yes. A very natural alternative is:

• Večeras planiramo učiti hrvatski. = We plan to study Croatian tonight.

Another possible version is:

• Plan je učiti hrvatski večeras.

But Plan je da večeras učimo hrvatski is perfectly good Croatian and sounds like The plan is for us to study Croatian tonight.