Jestli zítra nebude pršet, uděláme si u řeky malý piknik.

Yes. In this sentence, jestli means if:

• Jestli zítra nebude pršet... = If it doesn’t rain tomorrow...

In everyday Czech, jestli is very common for conditions. You may also see:

• pokud = if, often a bit more formal
• když = can also mean if in some contexts, though it very often means when

So jestli is a very natural choice here.

Because Czech separates the conditional clause from the main clause with a comma.

• Jestli zítra nebude pršet, uděláme si u řeky malý piknik.

The first part is the condition:

• Jestli zítra nebude pršet = If it doesn’t rain tomorrow

The second part is the result:

• uděláme si u řeky malý piknik = we’ll have a small picnic by the river

This is very similar to English when the if-clause comes first.

Pršet means to rain. It is an impersonal verb, so Czech usually uses it without a real subject, just like English it rains.

The future is formed with bude + infinitive:

• bude pršet = it will rain
• nebude pršet = it will not rain / it won’t rain

So:

• zítra bude pršet = it will rain tomorrow
• zítra nebude pršet = it won’t rain tomorrow

The negation goes on bude:

• bude → nebude

Because Czech often does not use a subject in weather expressions.

English says:

• It is raining
• It will rain

But Czech normally just says:

• Prší
• Bude pršet

There is no need for a dummy subject like English it.

Yes. Czech word order is flexible, and both are possible:

• Jestli zítra nebude pršet
• Jestli nebude zítra pršet

Both mean the same thing. The difference is mostly one of emphasis or rhythm.

• zítra earlier in the clause can highlight tomorrow
• putting nebude earlier can sound slightly more neutral in some contexts

The version in your sentence is completely natural.

Uděláme means we will make / do.

The little word si is the reflexive dative pronoun, and here it gives the sense of doing something for ourselves or enjoying it ourselves.

So:

• uděláme piknik = we’ll make/have a picnic
• uděláme si piknik = we’ll have ourselves a picnic / we’ll treat ourselves to a picnic

In natural English, you would usually just say we’ll have a picnic, but in Czech si adds a nice sense of personal enjoyment.

Yes, but the sentence would feel a little different.

• Uděláme si u řeky malý piknik = more natural, cozy, personal
• Uděláme u řeky malý piknik = possible, but less idiomatic in this context

With activities like this, si is very common because it suggests that the speakers are doing something pleasant for themselves.

The preposition u takes the genitive case.

• řeka = river
• genitive singular: řeky
• u řeky = by the river / near the river

So this is:

• u
  • genitive

This phrase usually means near something, not literally in it or on it.

Not necessarily. U řeky usually means by the river or near the river.

It suggests location close to the river, but it does not specify the exact spot. If you wanted to be more specific, Czech could use other expressions, for example:

• na břehu řeky = on the riverbank

So u řeky is a general, natural way to say by the river.

Because malý has to agree with piknik in gender, number, and case.

Here:

• piknik is masculine inanimate singular
• it is the direct object of uděláme si
• for masculine inanimate nouns, the accusative singular looks the same as the nominative singular

So:

• malý piknik = nominative singular
• malý piknik = accusative singular as well

That is why the form stays malý.

Because the verb ending already shows the subject.

• uděláme = we will do / make

The ending -áme tells you the subject is we. Czech often leaves subject pronouns out when they are clear from the verb form.

You could say my uděláme, but that would usually add emphasis:

• my uděláme = we will, as opposed to someone else

In a normal sentence, just uděláme is enough.

Yes, exactly. This is a normal real future condition:

• condition: Jestli zítra nebude pršet
• result: uděláme si u řeky malý piknik

Czech does not need a special tense pattern like English learners may expect from some other languages. It simply uses normal future forms where needed.

So the structure is straightforward:

• if
  • future situation
• future result

Yes. That is correct.

• Jestli = common and conversational
• Pokud = also if, often slightly more formal or neutral

Both work here. The meaning stays basically the same.

Yes. Piknik is a normal Czech word, borrowed long ago and fully used in modern Czech.

It behaves like a regular masculine noun:

• ten piknik
• malý piknik
• na piknik
• bez pikniku

So even though it originally comes from another language, it fits into Czech grammar in a normal way.