Learn Hardcore Czech: Můj syn má novou počítačovou hru, která je teď jeho nejoblíbenější. - My son has a new computer game which is now his favourite.

Můj – possessive adjective my, masculine singular nominative, agreeing with syn.
syn – noun son, masculine animate, singular nominative; the subject of the main clause.
má – has, 3rd person singular present of mít (to have).
novou – adjective new, feminine singular accusative, agreeing with hru.
počítačovou – adjective computer, feminine singular accusative, also agreeing with hru.
hru – noun game, feminine singular accusative; direct object of má.
která – relative pronoun which/that, feminine singular nominative, referring back to hra/hru. It is the subject of the relative clause.
je – is, 3rd person singular present of být (to be).
teď – adverb now.
jeho – possessive pronoun his, indeclinable form here; refers to můj syn.
nejoblíbenější – adjective most favourite / favourite, superlative of oblíbený; here feminine singular nominative, agreeing with the implied hra.

Why is it novou and počítačovou, not nová and počítačová?

Because hru is in the accusative case, and adjectives must agree with the noun in gender, number, and case.

The base form is nová počítačová hra (a new computer game) – nominative feminine singular.
In the sentence, this noun phrase is the direct object of má, so it goes into the accusative:
- nová → novou (feminine singular accusative)
- počítačová → počítačovou (feminine singular accusative)
- hra → hru (feminine singular accusative)

Pattern for many -a feminine nouns and adjectives:

Nominative: nová hra
Accusative: novou hru

So the whole object is correctly novou počítačovou hru.

Why is it hru and not hra?

Because hru is the direct object of the verb má (has), so it must be in the accusative case.

hra – nominative (subject form)
hru – accusative (object form)

Compare:

Hra je nová. – The game is new. (hra is the subject → nominative)
Mám novou hru. – I have a new game. (hru is the direct object → accusative)

In your sentence, Můj syn má novou počítačovou hru, hru is the thing that he has, so accusative is required.

What gender is hra, and how does that affect the other words?

Hra is feminine. This affects:

Adjectives before it
They must be feminine too:
- nová hra (new game)
- počítačová hra (computer game)
Case endings
In the accusative singular:
- nová → novou
- počítačová → počítačovou
- hra → hru
Relative pronoun referring to it
The relative pronoun must agree in gender and number:
- feminine singular → která (which)

So we get: Můj syn má novou počítačovou hru, která…

Why is the relative pronoun která, and not který, kterou, or something else?

The form of který/která/které depends on:

The gender and number of the noun it refers to.
- It refers to hra/hru, which is feminine singular → base form která.
The grammatical function inside the relative clause.
Inside the clause která je teď jeho nejoblíbenější:
- která is the subject (it stands for that game).
- The verb is je (is).
- For a subject, we use the nominative case → která (fem. sg. nominative).

Even though hru is accusative in the main clause, která has its own case based on its role in the relative clause, not the main one.

So:

Feminine singular nominative subject → která.
If it were an object in the clause, it might be kterou instead.

Why is there a comma before která? Could we leave it out?

In standard Czech, you must put a comma before a clause introduced by který/která/které when it follows a noun.

která je teď jeho nejoblíbenější is a subordinate (relative) clause that gives additional information about hru. Czech punctuation rules require a comma before such clauses:

Můj syn má novou počítačovou hru, která je teď jeho nejoblíbenější.

Unlike English, where commas around relative clauses depend on “restrictive vs non-restrictive” meaning, Czech is much more formal about this; here you simply put the comma. Dropping it would be considered a punctuation mistake.

Why is it jeho nejoblíbenější, not svou nejoblíbenější?

This involves the reflexive possessive svůj vs the normal possessive jeho.

svůj/svoje always refers to the subject of the same clause.
jeho means his in general and can refer to someone else, not necessarily the subject.

In the relative clause která je teď jeho nejoblíbenější:

The subject of this clause is která (hra).
The possessor you want is the son (můj syn), not the game.

If you used svou (feminine form of svůj):

která je teď svou nejoblíbenější
→ literally: which is now its own favourite, i.e. the game is its own favourite game, which is semantically wrong.

You need a possessive that can point back to můj syn, who is outside this clause, so you use jeho:

jeho nejoblíbenější = his favourite (the son’s favourite).

How is nejoblíbenější formed, and what’s the difference from oblíbená?

nejoblíbenější is the superlative form of oblíbený (liked, popular).

Formation steps:

oblíbený – liked, favorite (adjective)
oblíbenější – more liked, more favourite (comparative)
nejoblíbenější – most liked, favourite (superlative), formed by adding nej- to the comparative.

So:

oblíbená hra – a liked / popular game.
nejoblíbenější hra – the favourite game (out of all games).

In your sentence, nejoblíbenější is used substantively (without repeating hra) and means (the) favourite one. Using oblíbená instead would downgrade the meaning from the favourite to just liked:

která je teď jeho oblíbená → which is now one he likes (but not necessarily the top favourite).

Why can the sentence end with just nejoblíbenější? Where did hra go?

Czech, like English, can often omit a repeated noun when it is clear from context.

Earlier in the sentence, we already had novou počítačovou hru, so we know what we are talking about.

Full, explicit version:
- …která je teď jeho nejoblíbenější hra.
  or
- …která je teď jeho nejoblíbenější hrou. (instrumental, also common with být
  - noun)
Natural, shorter version:
- …která je teď jeho nejoblíbenější.
  → literally: which is now his most favourite (one).

The noun hra is simply understood and left out. This is very normal in Czech when the meaning is obvious.

Is it better to say jeho nejoblíbenější hra or just jeho nejoblíbenější?

Both are grammatical; the choice is about style and emphasis.

…která je teď jeho nejoblíbenější hra.
- Explicitly repeats hra.
- Slightly heavier / more formal-sounding.
- Very clear structure: which is now his favourite game.
…která je teď jeho nejoblíbenější hrou.
- Uses instrumental (hrou) as predicate.
- Also correct, often a bit more formal/“school-book” style.
…která je teď jeho nejoblíbenější.
- Most natural in everyday speech and writing.
- Avoids repeating the noun; feels fluent and idiomatic.
- Mirrors English “which is now his favourite”.

In normal conversation or neutral writing, the version in your sentence (3) is probably the best choice.

Can I move teď around in the relative clause? For example: která teď je jeho nejoblíbenější or která je jeho nejoblíbenější teď?

Yes, but the most neutral and common placement is the one you have:

která je teď jeho nejoblíbenější – neutral, natural.

Other options:

která teď je jeho nejoblíbenější
- Possible, but sounds a bit marked or emphatic, as if you were stressing teď (now, as opposed to earlier).
která je jeho nejoblíbenější teď
- Also possible, but sounds slightly unusual; can emphasize that this is true at this moment rather than generally.

In practice, native speakers overwhelmingly use která je teď jeho nejoblíbenější.

Could I say Můj syn má novou hru na počítač instead of novou počítačovou hru?

Yes, but there is a nuance:

novou počítačovou hru
- Uses the adjective počítačový (computer).
- This is the standard way to say computer game / video game as a general category.
- Sounds very natural: you are talking about a type of game.
novou hru na počítač
- Literally: a new game for the computer.
- Also correct and understood.
- Slightly more specific and concrete: a game that you play on a (this) computer, not necessarily a generic “computer game” category.

Both are fine, but novou počítačovou hru is the most typical phrasing for a new computer game in this kind of general sentence.

Can I change the word order of the main clause, like Novou počítačovou hru má můj syn?

Yes. Czech word order is flexible, and you can reorder elements to change emphasis.

Můj syn má novou počítačovou hru…
- Neutral, subject–verb–object order.
- Slight emphasis on můj syn (who has it).
Novou počítačovou hru má můj syn…
- Fronts the object, emphasising what he has: It is a new computer game that my son has…
- Still grammatical and idiomatic, just a different focus.

You should, however, keep the relative clause right after its noun (or noun phrase):

Novou počítačovou hru, která je teď jeho nejoblíbenější, má můj syn.
– Also correct, now strongly emphasising this particular game.

What’s the difference between teď, nyní, and words like momentálně here?

All can express a similar idea, but with different register or nuance:

teď – now
- Most common, neutral, everyday word.
- Fits perfectly here: která je teď jeho nejoblíbenější.
nyní – also now
- More formal, a bit bookish or used in official speech.
- která je nyní jeho nejoblíbenější sounds slightly more formal or written.
momentálně – at the moment / currently
- Often implies temporariness, maybe that this could change.
- která je momentálně jeho nejoblíbenější = which is currently his favourite (for now).

In your sentence, teď is the most natural and neutral choice.

Could I use co instead of která, like …hru, co je teď jeho nejoblíbenější?

Spoken Czech often does this, but it is considered colloquial, not standard:

Můj syn má novou počítačovou hru, co je teď jeho nejoblíbenější.
- Very common in informal speech.
- Not appropriate in formal writing or when aiming for standard grammar.

In standard Czech, you should use která to refer back to hra:

Můj syn má novou počítačovou hru, která je teď jeho nejoblíbenější.

So: co is fine in casual conversation, která is the correct form for careful speech and writing.