Můj kamarád si bude hrát s námi v parku.

Because kamarád is a masculine animate singular noun, and můj must agree with it in gender, number, and case.

• můj kamarád = my male friend
• moje kamarádka = my female friend

So můj is the correct nominative singular form for a masculine noun like kamarád.

In hrát si, the word si is part of the verb phrase. The full expression hrát si means to play, often in the sense of to play around, to have fun, or to play games/toys, especially like children do.

Compare:

• hrát = to play something, or to perform
  
  • hrát fotbal = to play football
  • hrát na klavír = to play the piano
• hrát si = to play, to be playing around, to amuse oneself
  
  • Děti si hrají. = The children are playing.

So in this sentence, si is not really translated word-for-word as to himself; it is simply part of the natural Czech verb hrát si.

Because hrát si is an imperfective verb. In Czech, the future of imperfective verbs is usually formed with:

• a future form of být (budu, budeš, bude...)
• plus the infinitive

So:

• bude si hrát = he will play

This is the normal future formation for imperfective verbs.

By contrast, many perfective verbs form the future with a single present-looking form. But hrát si is imperfective, so Czech uses bude + infinitive.

Both can be encountered, but si bude hrát reflects an important Czech word-order rule: short unstressed words like si, se, mi, ti, ho, etc. often go in the second position in the sentence. These are called clitics.

In:

• Můj kamarád si bude hrát s námi v parku.

the first unit is Můj kamarád, and si comes right after it, in second position.

So this placement is very natural Czech word order.

S meaning with requires the instrumental case.

The pronoun my (we) changes in the instrumental to námi:

• my = we
• s námi = with us

So:

• s námi = with us

This is a very common pattern in Czech:

• s kamarádem = with a friend
• s rodiči = with parents
• s námi = with us

Because the preposition v meaning in or inside usually takes the locative case when it describes location.

The noun park changes in the locative singular:

• park = park
• v parku = in the park

So the sentence uses the correct case after v.

It is the locative singular.

For the masculine inanimate noun park, the locative singular form is:

• nominative: park
• locative: v parku

This is used after prepositions like v and na when talking about location.

It is less natural. The biggest issue is the position of si.

In normal Czech, si prefers the second position, so:

• Můj kamarád si bude hrát s námi v parku. = natural
• Můj kamarád bude si hrát... = much less natural / not the standard choice

So for learners, it is best to use si early in the sentence, usually in second position.

Yes. Kamarád is grammatically masculine and usually refers to a male friend.

The female form is:

• kamarádka = female friend

So:

• Můj kamarád = my male friend
• Moje kamarádka = my female friend

In some contexts, English friend is gender-neutral, but Czech usually makes the distinction.

It is used most naturally for children playing, but it can also be used for adults in some contexts.

Common uses:

• children playing in a park
• someone playing around with an object
  
  • hraje si s telefonem = he is fiddling/playing with the phone
• playful or figurative uses

In this sentence, the most natural interpretation is probably that the friend is playing in a fun, childlike sense.

Yes. Czech word order is fairly flexible, and different orders can shift emphasis.

For example:

• Můj kamarád si bude hrát s námi v parku.
  Neutral: My friend will play with us in the park.
• V parku si bude hrát s námi můj kamarád.
  Emphasis on in the park
• S námi si bude hrát můj kamarád v parku.
  Emphasis on with us

Even though the order can change, the cases still show the grammatical roles. For learners, the original sentence is a very good neutral pattern.

Because Czech has no articles like a or the.

So park or v parku can mean:

• in a park
• in the park

The exact meaning depends on context.

This is a major difference from English: Czech normally leaves definiteness unstated unless context makes it clear.