Dešťová voda je v konvi.

Czech commonly expresses this idea as adjective + noun:

• dešťová = rain / rain-related
• voda = water

So dešťová voda means rainwater. English often prefers a compound noun, but Czech often uses an adjective in this kind of phrase.

Because it has to agree with voda.

In Czech, adjectives agree with the noun they describe in:

• gender
• number
• case

Here, voda is:

• feminine
• singular
• nominative

So the adjective must also be feminine singular nominative: dešťová.

It is in the nominative case because it is the subject of the sentence — the thing that is somewhere.

Since dešťová describes voda, it is also in the nominative.

Je means is.

It is the 3rd person singular present tense of the verb být = to be.

So:

• je = is

In this sentence, it links the subject and the location:

• Dešťová voda je v konvi.
• The rainwater is in the can.

Because after v meaning in, Czech normally uses the locative case for location.

The dictionary form is konev, but after v it changes to the locative singular:

• konev = dictionary form
• v konvi = in the can

So konvi is not a different word; it is a different case form of the same noun.

The dictionary form is konev.

So if you look it up, you would search for konev, not konvi.

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

So Dešťová voda je v konvi could mean:

• The rainwater is in the can
• Rainwater is in a can
• The rainwater is in a can

The exact meaning depends on context.

Yes. Czech word order is fairly flexible.

The sentence Dešťová voda je v konvi is a natural, neutral way to say it. But you could also say:

• V konvi je dešťová voda.

Both are grammatical. The difference is mainly emphasis:

• Dešťová voda je v konvi. → neutral, starting with the subject
• V konvi je dešťová voda. → puts more focus on where it is

You would not use v konvi.

For motion into something, Czech usually uses do + genitive:

• v konvi = in the can (location)
• do konve = into the can (movement)

So:

• Dešťová voda je v konvi. = The rainwater is in the can.
• Dešťová voda teče do konve. = The rainwater is flowing into the can.

Yes, very important.

Czech diacritics are part of the spelling, not optional decorations.

For example:

• š is pronounced like sh
• ť is a soft t
• á shows a long vowel

So you should write:

• dešťová, not destova
• konvi, not konvy

Leaving off the marks can make the word incorrect or harder to understand.

Not exactly in the usual English sense.

In this phrase, dešťová means something like rain-related or from rain. So dešťová voda means water from rain = rainwater.

So although dešťový / dešťová / dešťové can sometimes be translated as rainy in some contexts, here the best translation is rainwater, not rainy water.