Moje sestra se bojí plavat v řece, ale u moře je klidná.

Se bojí means is afraid or is scared.

The dictionary form is bát se, which means to be afraid.

• bát se = to be afraid
• bojí se = he/she is afraid

In this sentence, sestra se bojí means my sister is afraid.

Here, se does not literally mean herself. It is part of the verb bát se.

So you should learn bát se as a whole expression, not just bát.

This is common in Czech:

• bát se = to be afraid
• smát se = to laugh
• dívat se = to look/watch

In many cases, se is just a required part of the verb.

Both words belong together, but Czech clitics like se usually go in the second position of the clause.

So:

• Moje sestra se bojí... is the normal order.
• Bojí se... is also possible when the verb comes first.
• Moje sestra bojí se... sounds unnatural in normal Czech.

So the placement of se is mostly about Czech word-order rules, not about meaning.

Because sestra is a feminine noun, the possessive adjective must agree with it.

The possessive můj changes form:

• můj bratr = my brother
• moje sestra / má sestra = my sister
• moje auto = my car

So moje matches the feminine noun sestra here.

Also, má sestra is possible too. Moje sestra is very common in everyday speech.

After bát se, Czech often uses an infinitive to say what someone is afraid to do.

So:

• bojí se plavat = she is afraid to swim / afraid of swimming

This works much like English:

• She is afraid to swim
• She is afraid of swimming

In Czech, the infinitive plavat is a very natural choice here.

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

The noun řeka changes like this:

• řeka = river
• v řece = in the river

So:

• v = in
• řece = locative singular of řeka

Also remember that Czech has no articles, so v řece can mean in the river or in a river, depending on context.

U moře means by the sea, near the sea, or at the seaside.

It does not mean in the sea.

Compare:

• u moře = by/near the sea
• v moři = in the sea

So the sentence contrasts these two ideas nicely:

• she is afraid to swim in a river
• but by the sea / at the seaside she is calm

That does not necessarily mean she is swimming in the sea. It just says she is calm when she is there.

Because u takes the genitive case, and the genitive singular of moře happens to be moře.

So:

• moře = sea
• u moře = by the sea

This can be confusing because the form looks unchanged.

For comparison:

• nominative: moře
• genitive: moře
• locative: moři

So:

• u moře is correct
• v moři would be a different preposition and a different case

Because klidná is an adjective describing sestra, and Czech adjectives must agree with the noun in gender, number, and case.

Since sestra is feminine singular, the adjective is feminine singular too:

• klidný = calm (masculine basic form)
• klidná = calm (feminine)
• klidné = calm (neuter)

So:

• sestra je klidná = the sister is calm

If it were a brother, you would say:

• bratr je klidný

Czech often leaves out subject pronouns when they are clear from context.

So je klidná literally means:

• is calm

But the subject is understood from the first clause:

• Moje sestra ... ale u moře je klidná
• My sister ... but by the sea she is calm

Czech does this very often:

• Jsem doma. = I am at home.
• Je unavená. = She is tired.

You usually do not need to say ona unless you want emphasis.

Czech word order is more flexible than English word order.

Here, u moře is placed early to highlight the contrast with v řece:

• v řece = in the river
• u moře = by the sea

So the sentence structure helps emphasize:

• afraid in one setting
• calm in another setting

The conjunction ale means but, and it introduces that contrast.

The sound ř is one of the most famous difficult Czech sounds.

In řece, it is roughly like an r and a zh sound said together, but there is no perfect English equivalent.

A rough approximation of řece for an English speaker is:

• rzhe-tseh

But that is only approximate.

A few helpful points:

• ř is not the same as plain r
• c in Czech is pronounced ts
• e is like eh

So řece sounds approximately like rzheh-tseh.

If you cannot say ř perfectly at first, that is completely normal. It takes practice.