V obchodě dnes koupím okurku, dvě rajčata a láhev oleje.

Because v meaning in / at a location requires the locative case.

• base noun: obchod
• locative singular: obchodě

So v obchodě means in the shop / at the shop.

This is a very common pattern in Czech:

• v domě = in the house
• v autě = in the car
• v obchodě = in the shop

The ending -ě is just the locative singular form here.

Because koupím comes from the perfective verb koupit.

In Czech, perfective verbs use present-tense endings to express a future completed action. So:

• koupím = I will buy
• not I am buying

If you want an ongoing or habitual meaning, Czech uses the imperfective verb:

• kupuju / kupuji = I buy / I am buying
• budu kupovat = I will be buying

So in this sentence, koupím means something like I’ll buy or I will buy.

Because it is the direct object of the verb koupím, so it goes into the accusative case.

• nominative: okurka
• accusative: okurku

This is normal for feminine nouns ending in -a:

• kniha → knihu
• taška → tašku
• okurka → okurku

So koupím okurku = I will buy a cucumber.

Because rajče is a neuter noun.

For the numeral 2, Czech uses:

• dva with masculine nouns
• dvě with feminine and neuter nouns

So:

• dva banány = two bananas
• dvě knihy = two books
• dvě rajčata = two tomatoes

That is why dvě is correct here.

Because nouns after 2, 3, 4 usually appear in the plural.

The noun rajče has:

• singular: rajče
• plural: rajčata

So:

• jedno rajče = one tomato
• dvě rajčata = two tomatoes

In this sentence, dvě rajčata is the correct form for two tomatoes.

Because Czech often uses the genitive after a container or quantity word.

Here, láhev means bottle, and oleje means of oil.

So the structure is:

• láhev = bottle
• oleje = of oil

Together: láhev oleje = a bottle of oil

This is a very common pattern:

• sklenice vody = a glass of water
• hrnek čaje = a cup of tea
• kilo cukru = a kilo of sugar
• láhev oleje = a bottle of oil

So olej changes to oleje because it depends on láhev.

Because only the main noun of the phrase is the direct object.

In láhev oleje:

• láhev is the thing being bought, so it is the object
• oleje is only specifying what kind of bottle it is: a bottle of oil

So the phrase works like this:

• object in accusative: láhev
• dependent noun in genitive: oleje

This is similar to English structure:

• a bottle of oil
• a cup of tea
• a piece of cake

The whole phrase is the object, but the internal grammar of the phrase still matters.

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

So a noun by itself can mean:

• a cucumber
• the cucumber

The exact meaning usually comes from context.

In this sentence:

• okurku could mean a cucumber
• láhev oleje could mean a bottle of oil

Czech speakers usually do not need articles to understand the sentence.

No. Czech word order is fairly flexible.

This sentence is natural, but you could also say:

• Dnes koupím v obchodě okurku, dvě rajčata a láhev oleje.
• Okurku, dvě rajčata a láhev oleje dnes koupím v obchodě.

The meaning stays similar, but the focus or emphasis changes.

The original order sounds natural because it starts with background information:

• V obchodě = where
• dnes = when
• koupím... = what will happen

So it is a good, normal Czech sentence, but not the only possible order.

Mostly yes: they are all part of what is being bought, so they function as objects of koupím.

• okurku = accusative
• dvě rajčata = accusative phrase
• láhev oleje = accusative phrase, but with oleje in genitive inside the phrase

So from the learner’s point of view:

• the list after koupím is a list of objects
• but the last item contains an extra genitive construction: bottle of oil

That is why the sentence mixes straightforward object forms with a noun phrase inside another noun phrase.