Kad sam umorna, teško se koncentriram.

Because umorna is the feminine singular form of the adjective umoran (tired).

In this sentence, the speaker is saying I am tired, and Croatian adjectives must agree with the person they describe in gender and number.

• Ja sam umorna = I am tired. (said by a woman)
• Ja sam umoran = I am tired. (said by a man)

So this sentence is being said by a female speaker.

Croatian often omits subject pronouns when they are clear from the verb.

Here, sam means am, and koncentriram means I concentrate / I am concentrating, so the subject ja (I) is understood.

So:

• Kad sam umorna, teško se koncentriram.
• Kad ja sam umorna... ❌

You can say ja for emphasis, but normally it is left out:

• Kad sam umorna, teško se koncentriram. = normal
• Kad sam ja umorna, teško se koncentriram. = more emphatic, like when I am tired...

Yes. Kad and kada both mean when.

In everyday speech, kad is very common and slightly shorter/more conversational. Kada can sound a bit more formal or neutral depending on context.

So these are both possible:

• Kad sam umorna, teško se koncentriram.
• Kada sam umorna, teško se koncentriram.

Both mean the same thing here.

Sam is the normal short (clitic) form of jesam (I am).

Croatian usually uses the short form in ordinary sentences:

• Ja sam umorna. = I am tired.

The full form jesam is mainly used for emphasis, contrast, or certain special structures:

• Jesam umorna, ali moram raditi. = I really am tired, but I have to work.

So in your sentence, sam is the natural choice.

Because sam is a clitic, and Croatian clitics usually go in the second position of their clause.

In the clause Kad sam umorna:

• Kad = first element
• sam = clitic in second position
• umorna = comes after that

This is a very typical Croatian word order pattern.

That is why:

• Kad sam umorna... ✅

and not usually:

• Kad umorna sam... ❌

Here, teško is an adverb meaning with difficulty, hard, or it is hard to.

So:

• teško se koncentriram = I have difficulty concentrating / I concentrate with difficulty

Be careful: it does not mean English hardly.

Compare:

• Teško radim kad sam umorna. = I work with difficulty when I’m tired.
• Teško razumijem. = I understand with difficulty.

Because the Croatian verb is koncentrirati se = to concentrate.

It is a reflexive verb, or at least a verb that normally appears with se in this meaning. So se is part of the usual verb form.

• koncentrirati se = to concentrate
• koncentriram se = I concentrate / I am concentrating

So:

• Teško se koncentriram. ✅
• Teško koncentriram. ❌ (not natural for this meaning)

A lot of Croatian verbs work this way, and learners often just need to memorize them together with se.

Because se is also a clitic, and clitics tend to come early in the clause, often after the first stressed element.

Here the natural order is:

• teško se koncentriram

This sounds normal and neutral.

You may also hear other word orders in real speech depending on emphasis, but for a learner, teško se koncentriram is the safest and most natural pattern.

It is present tense, first person singular:

• (ja) koncentriram se = I concentrate / I am concentrating

In Croatian, the present tense can express:

1. a general truth or habit

   • Kad sam umorna, teško se koncentriram.
     = When(ever) I’m tired, I have trouble concentrating.

2. a present situation, depending on context

   • Sad se teško koncentriram.
     = I’m having trouble concentrating now.

In your sentence, it sounds like a general repeated situation.

It can feel like whenever in English, even though Croatian just uses kad.

So the sentence can be understood as:

• When I’m tired, I have trouble concentrating.
• Whenever I’m tired, I have trouble concentrating.

Because it describes a general pattern, English often prefers when or whenever depending on style. Croatian does not need a separate special word here.

Not as standard neutral Croatian.

With reflexive se, the natural position is usually before the main verb here:

• Kad sam umorna, teško se koncentriram. ✅

Putting se after the verb:

• Kad sam umorna, teško koncentriram se. ❌ / very unnatural

So it is best to learn the chunk:

• teško se koncentriram

It is nominative singular feminine.

Even though the subject ja is omitted, the adjective still agrees with it:

• (ja) sam umorna

After forms of to be (biti), Croatian typically uses the nominative for predicate adjectives like this.

So:

• Ja sam umorna.
• Ti si umorna.
• Ona je umorna.

All of those use the nominative adjective form.

Only the adjective would change:

• Kad sam umoran, teško se koncentriram.

Everything else stays the same.

Other examples:

• feminine singular: umorna
• masculine singular: umoran
• plural mixed/masculine: umorni
• plural feminine: umorne

So the sentence clearly shows the speaker’s gender through umorna.