Záruka je platná i při poruše.

Because platná is an adjective and it has to agree with záruka.

• záruka = a feminine singular noun
• so the adjective must also be feminine singular
• the basic dictionary form is platný = valid
• in this sentence it changes to platná

So:

• platný = masculine
• platná = feminine
• platné = neuter

Since záruka is feminine, Czech uses platná.

Here i means also, even, or too, depending on how you translate it into natural English.

In this sentence, i při poruše most naturally means something like:

• even in the event of a failure
• also in case of malfunction

So i adds the idea that the warranty remains valid in that situation as well.

Compare:

• Záruka je platná při poruše. = The warranty is valid in the event of a failure.
• Záruka je platná i při poruše. = The warranty is valid even / also in the event of a failure.

Because the preposition při requires the locative case.

The noun is:

• porucha = failure, fault, malfunction

After při, it changes to the locative singular:

• porucha → při poruše

This is just a normal case change in Czech.

A few similar examples:

• při práci = during work
• při nehodě = in an accident / during an accident
• při problému = in the event of a problem

So při poruše is correct because při + locative is the rule.

Poruše is locative singular.

The noun is:

• nominative: porucha
• locative singular: (při) poruše

This is a common pattern for feminine nouns ending in -a:

• škola → ve škole
• firma → ve firmě
• porucha → při poruše

So the ending changes from -a to -e or -ě, depending on the word.

Literally, při often means during, at, or in the course of. But in practical usage, při poruše here is best understood as:

• in the event of a failure
• in case of malfunction
• when a failure occurs

So although a word-for-word translation might suggest during a malfunction, the natural meaning in this kind of sentence is broader: the warranty still applies if the product breaks or malfunctions.

Je is the 3rd person singular of být = to be.

So:

• záruka = the warranty
• je = is
• platná = valid

Together: Záruka je platná = The warranty is valid

In present-tense Czech, být is often omitted in some informal contexts, but not here.
You normally need je in this kind of standard statement.

So:

• Záruka je platná. = correct
• Záruka platná. = not normal standard Czech for this meaning

Yes, Czech word order is flexible, but the original order is the most neutral and natural.

Standard version:

• Záruka je platná i při poruše.

You could rearrange parts for emphasis, for example:

• I při poruše je záruka platná.

This puts more emphasis on i při poruše = even in the event of a failure.

So the meaning stays very similar, but the focus changes slightly.

For a learner, the original sentence is the best neutral pattern to remember.

Not always. Záruka can often be translated as either:

• warranty
• guarantee

In commercial and product contexts, warranty is often the better English choice.
So in this sentence, The warranty is valid even in the event of a failure is probably the most natural translation.

English guarantee is broader, while Czech záruka is very commonly used for product warranty coverage.

Yes, it is natural. But depending on the context, a Czech speaker might also say:

• Záruka platí i při poruše.
• Záruka je platná i v případě poruchy.

These are very close in meaning.

A useful distinction:

• je platná = is valid
• platí = applies / is valid
• v případě poruchy = in case of failure

So the original sentence is correct and natural, but there are other common ways to express the same idea.

Because Czech has no articles.

English distinguishes between:

• a warranty
• the warranty

Czech simply says:

• záruka

The exact meaning depends on context. In this sentence, English usually needs the warranty, but Czech does not mark that difference with an article.

That is completely normal in Czech:

• Auto je drahé. = The car is expensive / A car is expensive.
• Záruka je platná. = The warranty is valid.

The listener understands from context whether it is definite or indefinite.