Kajak se převrhne, ledaže posádka začne pádlovat.

Here se is not really itself in the literal English sense. It is a very common Czech reflexive particle that often helps form verbs with meanings like:

• something happens by itself
• something changes state
• something happens to the subject without naming an external agent

So kajak se převrhne means the kayak will capsize / tip over.

Compare:

• Převrhnout kajak = to overturn a kayak (someone overturns it)
• Kajak se převrhne = the kayak overturns / will overturn

In this sentence, se makes the event intransitive: the kayak is the thing that ends up upside down, and we are not focusing on who caused it.

This is a very important Czech point: perfective verbs use present-tense forms to express the future.

The verb here is převrhnout se / převrhnout in the perfective sense: a single completed event of overturning. Because it is perfective, the form převrhne does not mean ordinary present is overturning. It means will overturn / will capsize.

So:

• kajak se převrhne = the kayak will capsize
• not the kayak is capsizing in the ongoing sense

If you wanted an ongoing present idea, Czech would normally use an imperfective verb.

The same thing happens in začne:

• začne pádlovat = will start paddling

Ledaže means unless.

So the sentence structure is:

• Kajak se převrhne = the kayak will capsize
• ledaže posádka začne pádlovat = unless the crew starts paddling

It introduces an exception to what would otherwise happen.

It is close in meaning to:

• pokud ne = if not
• jestli ne = if not

But ledaže is a single conjunction meaning unless, and it can sound a bit more formal or literary than the most everyday alternatives.

In Czech, subordinate clauses are normally separated by commas.

Since ledaže posádka začne pádlovat is a subordinate clause introduced by the conjunction ledaže, Czech punctuation requires a comma before it:

• Kajak se převrhne, ledaže posádka začne pádlovat.

This is normal Czech punctuation, even where English punctuation might sometimes feel lighter.

Začne pádlovat means will start paddling or begins to paddle.

This emphasizes the start of the action. The verb začít means to begin / to start, and it is followed by an infinitive:

• začít pádlovat = to start paddling

If you said posádka pádluje, that would mean the crew is paddling / paddles, which is a different idea.

So the sentence is not just saying that paddling matters in general. It is saying that the kayak will capsize unless the crew starts paddling.

Both kajak and posádka are in the nominative singular, because each is the subject of its clause.

• Kajak se převrhne
  kajak = subject, nominative singular

• ledaže posádka začne pádlovat
  posádka = subject, nominative singular

So this sentence is structurally quite straightforward:

• subject + verb
• conjunction + subject + verb + infinitive

Posádka is grammatically singular, even though it refers to a group of people: crew.

It is a feminine noun in Czech.

That is why the verb is singular:

• posádka začne = the crew will start

This is very similar to English the crew starts.

If you wanted to refer to the individual people more explicitly, you could use a plural noun, but posádka itself is singular.

The main finite verbs are perfective:

• převrhne se ← from převrhnout se (perfective)
• začne ← from začít (perfective)

Perfective verbs in Czech usually express:

• a single completed event
• the beginning or completion of something
• future meaning when used in present-tense forms

The infinitive pádlovat is imperfective, because it refers to the activity itself, not a completed single result.

So the sentence combines them very naturally:

• začne = the beginning of an action, perfective
• pádlovat = the ongoing activity, imperfective

That is exactly the combination Czech usually wants after začít.

Yes, Czech word order is more flexible than English, but different orders change emphasis.

The neutral order here is:

• Kajak se převrhne, ledaže posádka začne pádlovat.

You could also place the subordinate clause first:

• Ledaže posádka začne pádlovat, kajak se převrhne.

This still means the same thing, but the emphasis shifts slightly toward the condition/exception first.

Within the clause, changing word order is also possible, but the original is the most natural and neutral for a learner to use.

They are related, but not always identical.

• převrhnout se = to overturn / capsize / tip over
• překlopit se = to tip over / flip over
• obrátit se = to turn / turn over, but often broader and less specific

For a kayak, převrhnout se is a very natural choice if you mean capsize. It strongly suggests that the kayak ends up overturned.

So in this context, převrhne se is a good, idiomatic verb.

A rough pronunciation guide is:

Kajak se převrhne, ledaže posádka začne pádlovat.

Approximate English-style help:

• Kajak ≈ KAI-yak
• se ≈ seh
• převrhne ≈ tricky, roughly PRZHE-vr-hneh
• ledaže ≈ LEH-da-zheh
• posádka ≈ po-SAAT-ka
• začne ≈ ZA-chneh
• pádlovat ≈ PAA-dlo-vat

A few sounds are hard for English speakers:

• ř in převrhne is the famous Czech sound, something like a mix of r and zh
• ž in ledaže sounds like the s in measure
• long vowels matter: á is longer than a

Yes. That is a very natural alternative.

• Pokud posádka nezačne pádlovat, kajak se převrhne.
• If the crew does not start paddling, the kayak will capsize.

This version uses a regular if clause with negation, instead of unless.

So:

• Kajak se převrhne, ledaže posádka začne pádlovat.
• Pokud posádka nezačne pádlovat, kajak se převrhne.

These are very close in meaning. The first uses unless; the second uses if not.