V obchodě jsou dnes levná rajčata, okurky a cibule.

Because v meaning in (for a location) requires the locative case.

The noun obchod changes like this:

• obchod = nominative singular
• v obchodě = locative singular

So v obchodě means in the shop/store.

A useful contrast:

• v obchodě = in the shop
• do obchodu = into the shop

Jsou is the 3rd person plural form of být (to be).

It is plural because the subject is plural: rajčata, okurky a cibule.

Compare:

• je = is
• jsou = are

So the sentence uses jsou because it is talking about multiple things.

In form, levná agrees with rajčata, the first noun in the list.

• rajče is a neuter noun
• rajčata is nominative plural
• the matching adjective form is levná

So:

• levná rajčata = cheap tomatoes

This can feel strange to English speakers, because one adjective is placed before a whole list. In Czech, that is normal.

It is understood to describe all three items.

Czech often puts one adjective before a list, even if the later nouns would technically need different adjective endings if the adjective were repeated.

If you repeated the adjective with each noun, you would get:

• levná rajčata
• levné okurky
• levné cibule

So in the original sentence, levná is written once, but the meaning extends to the whole list.

That is a very good question. Cibule can look the same in the nominative singular and nominative plural.

So:

• cibule = onion
• cibule = onions

In this sentence, it is naturally understood as plural because it appears in a list of vegetables for sale.

If you repeated the adjective, the plural would become clearer:

• levné cibule = cheap onions

Yes. They are the subject of the sentence, so they are in the nominative.

Even though English often says there are..., Czech does not need a separate dummy subject like there. The actual things being talked about stay in the nominative:

• rajčata
• okurky
• cibule

Because Czech does not use a dummy subject like English there in this type of sentence.

English says:

• There are cheap tomatoes in the shop.

Czech simply says, more literally:

• In the shop are cheap tomatoes...

So jsou works without any extra word meaning there.

Yes, Czech word order is fairly flexible.

The sentence:

• V obchodě jsou dnes levná rajčata, okurky a cibule.

is natural, but other orders are possible too, for example:

• Dnes jsou v obchodě levná rajčata, okurky a cibule.
• Levná rajčata, okurky a cibule jsou dnes v obchodě.

The basic meaning stays the same, but the focus changes:

• V obchodě... puts attention on the place
• Dnes... puts attention on today
• starting with the vegetables puts attention on what is cheap

Because Czech has no articles.

So Czech does not normally mark the difference between:

• a shop / the shop
• cheap tomatoes / the cheap tomatoes

You understand that from context instead.

That is why Czech can simply say:

• v obchodě
• levná rajčata

without any word corresponding to a or the.

It is not completely irregular, but it is a form learners often have to memorize.

The singular is:

• rajče = tomato

The plural is:

• rajčata = tomatoes

This is a common neuter pattern for some nouns ending in -e.

So:

• rajče → rajčata

That is why the adjective is also in the neuter plural form:

• levná rajčata

They are:

• rajče = tomato
• okurka = cucumber
• cibule = onion

Their plural forms here are:

• rajčata = tomatoes
• okurky = cucumbers
• cibule = onions

So the sentence combines three different noun patterns, which is one reason it is useful for practice.

Normally, you say v obchodě.

Czech sometimes uses ve instead of v to make pronunciation easier, especially before difficult consonant clusters:

• ve škole
• ve městě

But obchodě does not create that kind of problem, so v obchodě is the normal form here.