Oba spoluhráči jsou v pokoji.

Oba means both.

That is the key difference:

• oba spoluhráči = both teammates
• dva spoluhráči = two teammates

So oba refers to a specific pair taken together, while dva just gives the number.

Also, oba is used here because spoluhráči is masculine animate plural.

Because the subject is plural and in the nominative case.

The basic singular form is:

• spoluhráč = teammate

Its nominative plural is:

• spoluhráči = teammates

In this sentence, oba spoluhráči is the subject, so Czech uses the nominative plural.

Jsou is the 3rd person plural form of být (to be).

Because the subject is plural (both teammates), the verb must also be plural:

• je = is
• jsou = are

So:

• spoluhráč je = the teammate is
• spoluhráči jsou = the teammates are

Because the preposition v changes the case.

Here v means in, expressing location, and with that meaning it requires the locative case.

So:

• dictionary form: pokoj = room
• locative singular: v pokoji = in the room

This is very common in Czech:

• v domě = in the house
• v autě = in the car
• v pokoji = in the room

It is locative singular.

The noun is pokoj (room), and after v when talking about location, Czech uses the locative:

• pokoj → base form / nominative
• v pokoji → locative

So v pokoji literally means something like in room but with the Czech locative ending.

Not always.

With v, the case depends on the meaning:

• v + locative = location, in/inside/at
• v + accusative = motion into something

Examples:

• Jsou v pokoji. = They are in the room. → location → locative
• Jdou do pokoje. = They are going into the room. → motion inward, but Czech usually uses do + genitive here rather than v + accusative

In modern Czech, for movement into a room, do pokoje is the normal choice.

Because Czech does not have articles like English a and the.

So Czech simply says:

• pokoj = room / a room / the room

The exact meaning depends on context.

That means v pokoji can correspond to:

• in a room
• in the room

In your sentence, the intended meaning is understood from context.

Yes. Czech word order is quite flexible.

The neutral order here is:

• Oba spoluhráči jsou v pokoji.

But other orders are possible, for example:

• V pokoji jsou oba spoluhráči.
• Oba jsou v pokoji. (if it is already clear who they are)

Changing the order usually changes emphasis rather than the core meaning.

For example:

• V pokoji jsou oba spoluhráči. puts more focus on where they are
• Oba spoluhráči jsou v pokoji. is a neutral statement

Because Czech usually does not need subject pronouns when the subject is already stated or clear from the verb form.

Here the subject is explicitly given:

• Oba spoluhráči = both teammates

So there is no need to add a separate word for they.

Even without the noun phrase, jsou already tells you the verb is plural:

• Jsou v pokoji. = They are in the room. / You are in the room. (depending on context)

But in your sentence, the noun phrase makes the subject completely clear.

Not exactly, but its form depends on gender.

In the nominative:

• oba is used with masculine animate
• obě is used with feminine, neuter, and masculine inanimate

Examples:

• oba spoluhráči = both teammates (masculine animate)
• obě sestry = both sisters
• obě okna = both windows

So in your sentence, oba is correct because spoluhráči refers to masculine animate people.

Spoluhráč means teammate or fellow player.

It is built from:

• spolu- = together / co-
• hráč = player

So spoluhráč is literally something like co-player, meaning someone on the same team or playing together with someone else.

Yes, but the meaning changes slightly.

• oba hráči = both players
• oba spoluhráči = both teammates

Spoluhráči makes it clear that the two players are on the same side or team.
Hráči is more general and just means players.

Roughly like:

spo-loo-HRAA-chee

A few helpful points:

• spolu sounds like spo-loo
• hrá- has a long á
• č sounds like ch in chocolate
• the stress in Czech normally falls on the first syllable of the whole word: SPOLUhráči

So the main stress is at the start, even though English speakers may be tempted to stress hrá more strongly.