Když je řeka klidná, můžeme jet malou lodí k tomu mostu.

Když means when in the sense of whenever / if it happens that.

In this sentence, Když je řeka klidná sets the condition or situation first:

• Když je řeka klidná = When the river is calm
• můžeme jet... = we can go / travel...

So the whole structure is very natural in Czech:

• Když + clause, main clause

It is similar to English:

• When the river is calm, we can...

Je is the 3rd person singular form of být (to be).

So:

• řeka = river
• je = is
• klidná = calm

Together:

• řeka je klidná = the river is calm

Czech often works just like English here: noun + to be + adjective.

Because klidná has to agree with řeka.

In Czech, adjectives change to match the noun’s:

• gender
• number
• case

Řeka is:

• feminine
• singular
• nominative here

So the adjective must also be feminine singular nominative:

• masculine: klidný
• feminine: klidná
• neuter: klidné

That is why you get:

• řeka je klidná

Because řeka is the subject of the clause.

In řeka je klidná (the river is calm), the river is the thing being described, so Czech uses the nominative case for řeka.

You can think of it this way:

• Who/what is calm? → řeka
• therefore → nominative

Můžeme means we can and jet is the infinitive to go / travel by vehicle.

So:

• můžeme = we can
• jet = go/travel by some vehicle or means of transport

Together:

• můžeme jet = we can go / we can travel

This is a very common Czech pattern:

• moci + infinitive
• můžu jít = I can go
• můžeme jet = we can go/travel

This is a very common question, because English uses go for both.

In Czech:

• jít = to go on foot
• jet = to go by vehicle / by some means of transport
• plout = to sail / float / move on water

Here the sentence says malou lodí (by a small boat), so jet is natural because the idea is traveling by a vehicle-like means.

So:

• jít k mostu = go to the bridge on foot
• jet k mostu = go to the bridge by vehicle / boat / bike etc.

A learner may also wonder about plout. That verb is possible in water contexts, but jet is often used broadly for traveling by boat as a means of transport.

Because lodí is in the instrumental case, which is often used after a means of transport or instrument.

Here, malou lodí means:

• by a small boat
• literally something like with a small boat / using a small boat

The adjective must match the noun, so:

• nominative: malá loď
• instrumental: malou lodí

That is why both words change:

• malá → malou
• loď → lodí

Yes. In Czech, a common way to express the means of transport is with the instrumental and no preposition:

• jet autem = go by car
• jet vlakem = go by train
• jet lodí = go by boat

So:

• malou lodí = by a small boat

This is a very natural Czech structure.

Because the preposition k requires the dative case.

The basic dictionary forms are:

• ten most = that bridge / the bridge (nominative)

But after k (to, toward), both the article-like demonstrative and the noun change:

• ten → tomu
• most → mostu

So:

• k tomu mostu = to that bridge / toward that bridge

This is one of the most important things to remember in Czech: prepositions often require specific cases.

Not exactly.

k usually means:

• to
• toward
• up to
• in the direction of

So k tomu mostu suggests movement toward the bridge or up to the bridge.

By contrast, do usually means into or to the inside of something:

• do domu = into the house / home
• k domu = toward the house / up to the house

A bridge is not really something you go into, so k mostu is the natural choice.

Because the sentence begins with a subordinate clause:

• Když je řeka klidná = subordinate clause
• můžeme jet malou lodí k tomu mostu = main clause

In Czech, just like in English, a clause introduced by když is separated by a comma.

So the comma is required here.

Yes. Czech word order is more flexible than English word order, although not completely free.

The sentence as given is very natural:

• Když je řeka klidná, můžeme jet malou lodí k tomu mostu.

But Czech can move parts around for emphasis, for example:

• Když je řeka klidná, můžeme k tomu mostu jet malou lodí.
• Malou lodí můžeme jet k tomu mostu, když je řeka klidná.

The basic meaning stays similar, but the focus changes slightly.

For learners, it is best to keep the original order until you get more comfortable with Czech emphasis patterns.

Czech does not have articles like a/an/the.

So Czech often uses just the noun:

• řeka = river / the river
• most = bridge / the bridge

If Czech wants to be more specific, it can use a demonstrative such as ten / ta / to:

• ten most = that bridge / sometimes effectively the bridge in context

In your sentence:

• řeka has no article because Czech simply does not need one
• tomu mostu uses a demonstrative, which gives a more specific sense: to that bridge

In many contexts, ten / ta / to really does mean that.

However, in everyday Czech, these demonstratives are also used very often where English might simply say the.

So k tomu mostu can be understood as:

• to that bridge
• or in some contexts almost to the bridge

The exact nuance depends on the situation. If the bridge is already known or visible, Czech often uses ten-forms very naturally.

Sometimes, yes, especially in practical contexts.

Literally, když means when, but in real usage it can overlap with English if when the meaning is conditional:

• Když je řeka klidná, můžeme jet...

This can feel like:

• When the river is calm, we can go...
• or If the river is calm, we can go...

Both are close in meaning here. Czech uses když very naturally in this kind of sentence.