Learn Hardcore Croatian: Njegova koncentracija je večeras slabija, ali trener je strpljiv i objašnjava vježbe polako. - His concentration is weaker tonight, but the coach is patient and explains the exercises slowly.

In Croatian, possessive adjectives (njegov, njezin, moj, tvoj, etc.) must agree in gender, number, and case with the noun they describe.

koncentracija is feminine singular nominative.
So njegov (masculine) changes to the feminine form njegova.
Together: njegova koncentracija = his concentration (literally his-fem concentration).

If the noun were masculine, e.g. njegov auto (his car), you would use njegov.

How do I know that koncentracija is feminine?

Many Croatian nouns ending in -acija (from Latin-based words) are feminine, for example:

situacija (situation)
informacija (information)
organizacija (organization)

Generally:

Nouns ending in -a are very often feminine (though not always).
koncentracija ends in -a and patterns with other -cija / -acija words, so it’s feminine.

That’s why you get:

njegova koncentracija (feminine form njegova)
slabija koncentracija (feminine form slabija)

Why is it slabija and not slabije after koncentracija je večeras…?

slabija here is a comparative adjective describing the noun koncentracija:

Positive: slab – weak
Comparative (masc): slabiji
Comparative (fem): slabija
Comparative (neut): slabije

Because koncentracija is feminine, the adjective must match:

Njegova koncentracija je večeras slabija.
His concentration is weaker tonight.

slabije (neuter or adverbial form) would be used differently, for example:

On se večeras koncentrira slabije.
He is concentrating worse / less well tonight.
Here slabije modifies the verb, not a noun.

What exactly does večeras mean, and why is it used instead of something like ove večeri?

večeras is a single adverb meaning “this evening / tonight (in the evening)”.

večer = evening
ove večeri = this evening (literally of this evening, genitive), more formal or more explicit
večeras = idiomatic, short, very common in everyday speech

So:

Njegova koncentracija je večeras slabija.
His concentration is weaker this evening / tonight.

You would only say ove večeri if you wanted a slightly more formal or more emphatic phrasing. večeras is the normal, natural choice.

Can the word večeras be placed somewhere else in the sentence?

Yes. Croatian word order is fairly flexible. These are all grammatically correct, with slight differences in emphasis:

Njegova koncentracija je večeras slabija. (neutral, standard)
Večeras je njegova koncentracija slabija.
– Emphasis on tonight (contrasting with other times).
Njegova koncentracija je slabija večeras.
– A bit less neutral; often used in speech, still fine.

The usual, most neutral place in this sentence is exactly as given: … je večeras slabija.

Why is there a comma before ali?

In Croatian, when ali (but) connects two independent clauses, you must use a comma:

Njegova koncentracija je večeras slabija, ali trener je strpljiv…

Two clauses:

Njegova koncentracija je večeras slabija.
Trener je strpljiv i objašnjava vježbe polako.

Since both parts could stand alone as separate sentences, the comma before ali is required, just like in English (…, but …).

Why is the verb je used twice: Njegova koncentracija je… ali trener je strpljiv…? Could it be left out?

je is the 3rd person singular form of biti (to be). In standard Croatian, you normally do not drop it in sentences like these:

Njegova koncentracija je večeras slabija.
Trener je strpljiv.

Each clause has its own subject and its own copula (je), so it appears twice.
Omitting je is possible only in very elliptical contexts, like headlines, notes, or very informal speech:

Koncentracija večeras slabija, trener strpljiv. (headline style)

But in normal spoken or written sentences, you keep je.

Why is it trener je strpljiv and not je trener strpljiv, and is the order flexible?

The most neutral word order in Croatian is Subject – Verb – (other parts):

Trener je strpljiv. (Subject trener, verb je, predicate adjective strpljiv)

However, Croatian does allow:

Je li trener strpljiv? (for a yes/no question)
Strpljiv je trener. (emphasizing strpljiv or giving a stylistic twist)

But in a straightforward statement, Trener je strpljiv is the default and sounds the most natural.

What is the difference between objašnjava and objasni / objasniti?

This is about aspect:

objašnjavati – imperfective (ongoing, repeated, process)
- on objašnjava – he explains / he is explaining
objasniti – perfective (single, completed action)
- on će objasniti – he will explain / he will have explained

In the sentence:

trener je strpljiv i objašnjava vježbe polako
We want to show that the coach is (patiently) in the process of explaining or does so repeatedly / habitually, so the imperfective objašnjava is correct.

If you used objasni in the present (on objasni), it would sound like a single, completed event, and in many contexts the perfective present actually refers to the future:

On će ti objasniti vježbe. – He will explain the exercises to you.

Why is vježbe in that form, and which case is it?

vježbe here is accusative plural, functioning as the direct object of the verb objašnjava.

Base form: vježba – an exercise (nominative singular, feminine)
Nominative plural: vježbe
Accusative plural (for most feminine -a nouns): vježbe (same as nominative plural form)

So:

objašnjava što? – vježbe (what is he explaining? – the exercises)

Even though nominative plural and accusative plural look the same here, the role in the sentence (direct object) tells you it is accusative.

What does polako mean exactly, and how is it different from sporo?

polako is an adverb meaning primarily “slowly”, but it often also carries nuances of:

gently, carefully, without rushing, take your time

sporo also means “slowly”, but it tends to focus more on slowness as a speed, sometimes with a more negative feel (too slow, sluggish).

In this context:

…trener je strpljiv i objašnjava vježbe polako.
implies he explains them slowly and patiently, so that the learner can follow.
polako fits perfectly with strpljiv (patient).

Word order with polako is flexible:

objašnjava vježbe polako (most natural)
objašnjava polako vježbe (possible, but less common here)

Could the overall word order be different while keeping the same meaning?

Yes, Croatian allows several variants. All of these are grammatically correct, though they differ slightly in emphasis:

Njegova koncentracija je večeras slabija, ali trener je strpljiv i polako objašnjava vježbe.
Večeras je njegova koncentracija slabija, ali trener je strpljiv i objašnjava vježbe polako.
Njegova je koncentracija večeras slabija, ali trener je strpljiv i objašnjava vježbe polako.
(inserting je after the first stressed word is also common)

The given sentence is the most neutral and natural version, but you can move adverbs and je around a bit to change rhythm and emphasis without changing the basic meaning.