Kad sam koncentrirana, lakše učim hrvatski.

Kad means when.

Both kad and kada are correct in Croatian. The difference is mainly style:

• kad = shorter, very common in everyday speech
• kada = a bit more formal or careful

So both of these work:

• Kad sam koncentrirana, lakše učim hrvatski.
• Kada sam koncentrirana, lakše učim hrvatski.

Because Croatian often drops subject pronouns when they are already clear from the verb.

Here, učim means I learn / I am learning, so ja is not necessary.

• učim = I learn
• ja učim = I learn, but with extra emphasis on I

You would usually add ja only if you want contrast or emphasis, for example:

• Ja učim hrvatski, a on uči njemački.
  • I learn Croatian, and he learns German.

Sam is a short form of biti (to be), and short forms like this usually appear near the beginning of the clause.

So Kad sam koncentrirana is the natural order.

Think of it like this:

• kad = when
• sam = am
• koncentrirana = focused / concentrated

Croatian word order is more flexible than English, but these short words often have preferred positions. Kad sam koncentrirana sounds normal; Kad koncentrirana sam does not.

Because koncentrirana is the feminine singular form.

The speaker is female here, so the adjective agrees with her:

• Ja sam koncentrirana. = said by a woman
• Ja sam koncentriran. = said by a man

Croatian adjectives usually have to agree in gender and number with the person or thing they describe.

Here it functions as an adjective.

The structure is:

• sam = am
• koncentrirana = focused / concentrated

So sam koncentrirana literally means I am focused.

It is not the same as English I am concentrating. If you want that idea, Croatian would more naturally use a verb, for example:

• Kad se koncentriram... = When I concentrate...

So this sentence describes a state: being focused.

Yes, and it changes the nuance a little.

• Kad sam koncentrirana = When I am focused
  • describes your state
• Kad se koncentriram = When I concentrate
  • describes the action/process

Both are natural. The original sentence emphasizes the condition of being focused, not the act of focusing.

Lakše is the comparative adverb of lako.

• lako = easily
• lakše = more easily

So:

• učim = I learn
• lakše učim = I learn more easily

English often uses more easily, while Croatian often uses this one-word comparative form.

A useful comparison:

• On brzo uči. = He learns quickly.
• On brže uči. = He learns more quickly.

Because učiti and naučiti are different in aspect.

• učiti = to learn, study; an ongoing process
• naučiti = to learn/master something successfully; to finish learning

Here the sentence expresses a general habit or repeated situation:

• Kad sam koncentrirana, lakše učim hrvatski.
  • Whenever I am focused, I learn Croatian more easily.

That is why učim is the natural choice.

If you said naučim, it would suggest something more like I manage to learn / I succeed in learning, which is not the main idea here.

In Croatian, names of languages are normally written with a lower-case letter.

So:

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

This is different from English, where language names are capitalized.

So učim hrvatski is correct, not učim Hrvatski.

Yes, hrvatski is historically an adjective, but here it is used on its own to mean the Croatian language.

So:

• učim hrvatski = I am learning Croatian
• učim hrvatski jezik = I am learning the Croatian language

The shorter version, hrvatski, is very common and completely natural.

Grammatically, it is the direct object of učim, so it is in the accusative. For this masculine inanimate form, the accusative looks the same as the nominative, so you still see hrvatski.