Kamarádka Eva si bude hrát na zahradě a já budu sedět u okna.

Because hrát si and sedět are imperfective verbs. In Czech, the future of imperfective verbs is usually made with the future forms of být plus the infinitive:

• budu sedět = I will be sitting / I will sit
• bude si hrát = she will play / will be playing

This is different from perfective verbs, which usually form the future with their present-tense forms instead.

Si is a short reflexive word. In hrát si, it is part of the verb itself and does not literally mean to oneself in the usual English sense. The verb hrát si means to play, especially in the sense of playing around, having fun, playing as a child.

Compare:

• hrát = to play something, perform, play a game/instrument
• hrát si = to play, have fun, amuse oneself

So Eva si bude hrát means Eva will be playing.

Because si is a clitic, and Czech clitics usually go in the second position in the clause.

In Kamarádka Eva si bude hrát na zahradě:

• the first unit is Kamarádka Eva
• then the clitic si comes
• then the rest: bude hrát na zahradě

So this word order is very natural.

You may also hear different word orders in some contexts, but si tends to stay near the beginning of the clause, not just anywhere.

Kamarádka means female friend or girl friend in the non-romantic sense.

It is the feminine form of:

• kamarád = male friend
• kamarádka = female friend

So Kamarádka Eva means my/the friend Eva or Eva, a female friend.

This is a common Czech way to identify someone:

• kamarádka Eva = my/the friend Eva
• bratr Petr = brother Petr
• sestřenice Jana = cousin Jana

The noun gives the role or relationship, and the name tells you who the person is.

Because the preposition na here requires the locative case when it expresses location.

• zahrada = garden
• na zahradě = in/on the garden

In natural English, this is usually translated as in the garden, even though Czech uses na.

So:

• na zahradu = onto/to the garden area, toward the garden
• na zahradě = in the garden, on the garden area

Here the meaning is location, so na zahradě is correct.

Because Czech and English do not always use the same prepositions.

With some places, Czech uses:

• v = in
• na = on / at / in, depending on the expression

For garden, Czech normally says na zahradě. This is just the standard idiomatic choice.

So even though English says in the garden, Czech says na zahradě.

Because u means by, near, next to, while v means in.

• u okna = by the window / near the window
• v okně = in the window

If someone is sitting beside a window, Czech normally says sedět u okna.

Also, u takes the genitive case, so:

• okno → okna

That is why you see u okna.

Czech often does drop subject pronouns, because the verb ending already shows the person:

• budu sedět already means I will sit / I will be sitting

But já is often included for contrast or emphasis. In this sentence:

• Eva will be playing in the garden,
• and I will be sitting by the window.

So já helps stress the contrast between the two people.

Yes. You could say:

• Kamarádka Eva si bude hrát na zahradě a budu sedět u okna.

This is grammatically possible, because budu already shows I.

But the version with já sounds clearer and more contrastive, especially in a sentence comparing two people.

Either can work, depending on context.

• budu sedět can mean I will sit
• very often in natural English it is I will be sitting

Because sedět is imperfective, it often suggests an ongoing state rather than a single completed action.

So in this sentence, I will be sitting by the window is often the most natural English translation.

No, Czech word order is fairly flexible, but changing it can shift the emphasis.

The given sentence is natural and neutral:

• Kamarádka Eva si bude hrát na zahradě a já budu sedět u okna.

You could also move things around for emphasis, for example:

• Na zahradě si bude hrát kamarádka Eva...
• U okna budu sedět já.

But the basic meaning stays the same. Czech word order often reflects topic, focus, and emphasis, not just grammar.

They are different person forms of the future of být:

• budu = I will
• bude = he/she/it will

So:

• Eva si bude hrát = Eva will play
• já budu sedět = I will sit

The auxiliary has to agree with the subject.

The dictionary forms are:

• hrát si = to play
• sedět = to sit
• být = to be

In the sentence you see future constructions built from them:

• si bude hrát
• budu sedět

When learning Czech, it is important to remember hrát si as a full verb expression, not just hrát by itself.