Kupuję mrożone warzywa, jeśli nie mam czasu iść na rynek po świeże.

Kupuję is the imperfective form, and in the present tense it often describes a habit or something repeated.

So here it means something like:

I buy frozen vegetables if I don’t have time...

By contrast, kupię is perfective and usually refers to one completed future action:

I will buy frozen vegetables...

So kupuję is the natural choice for a general, repeated situation.

Because warzywa is the plural of warzywo (a vegetable / vegetable item), and the adjective has to agree with it.

• singular: mrożone warzywo would not work, because singular warzywo is neuter singular, so it would be mrożone warzywo
• plural: mrożone warzywa

Here both words are in the accusative plural, but for non-masculine-personal nouns like warzywa, the accusative plural looks the same as the nominative plural.

So:

• mrożone warzywa = frozen vegetables

It is plural.

• warzywo = a vegetable
• warzywa = vegetables

This can confuse English speakers because the Polish plural ending -a sometimes looks like a singular form, but here it is definitely plural.

In Polish, subordinate clauses are normally separated by a comma.

So in:

Kupuję mrożone warzywa, jeśli nie mam czasu...

the part starting with jeśli (if) is a subordinate clause, and Polish requires a comma before it.

This is more consistent than in English, where commas before if are often not used.

Jeśli means if, so it fits a conditional sentence very well.

• jeśli = if
• gdy = when / whenever
• kiedy = when

In this sentence, jeśli is the most direct choice because the meaning is conditional: if I don’t have time...

You could also hear gdy nie mam czasu in some contexts, especially if the meaning is closer to whenever I don’t have time, but jeśli is the clearest match for English if.

Polish often uses the pattern:

mieć czas + infinitive

which means to have time to do something.

So:

• mam czas iść = I have time to go
• nie mam czasu iść = I don’t have time to go

This is very natural Polish. English uses to before the second verb, but Polish usually just uses the infinitive directly.

You can also sometimes see a noun structure like nie mam czasu na pójście, but that is less natural here and sounds heavier.

Because Polish commonly uses na with places understood as open areas, events, or destinations such as markets.

So:

• iść na rynek = go to the market / marketplace

The preposition na here takes the accusative because it expresses movement toward a place.

By contrast, do is used with many enclosed places or institutions:

• do sklepu = to the shop
• do domu = home / to the house

With rynek in the sense of marketplace / market square, na rynek is the normal expression.

Here po means something like to get or for the purpose of getting.

A very common Polish pattern is:

iść / jechać / wpaść po + accusative

Examples:

• iść po chleb = to go get bread
• jechać po dzieci = to go pick up the children
• wyskoczyć po mleko = to pop out for milk

So:

iść na rynek po świeże
means
to go to the market for fresh ones

In full, it really means:

iść na rynek po świeże warzywa

The noun warzywa is omitted because it is already understood from the earlier part of the sentence.

Because Polish often leaves out a noun when it is obvious from context.

Earlier we already have mrożone warzywa, so later świeże clearly means:

świeże warzywa = fresh vegetables

This is very natural. English does the same sometimes:

I buy frozen vegetables if I don’t have time to go to the market for fresh ones.

So świeże is functioning almost like fresh ones.

Because po in this meaning takes the accusative, and the omitted noun is warzywa, which is non-masculine-personal plural.

For adjectives with that kind of plural noun, the accusative plural looks the same as the nominative plural:

• świeże warzywa
• mam świeże warzywa
• idę po świeże warzywa

So even though po requires the accusative, the form still comes out as świeże.

It comes historically from a verb, but in this sentence it behaves like an adjective.

• verb: mrozić = to freeze
• adjective/participle: mrożony = frozen

In everyday language, words like mrożony, gotowany, pieczony, smażony are often treated just like ordinary adjectives:

• mrożone warzywa = frozen vegetables
• gotowane ziemniaki = boiled potatoes
• pieczony kurczak = roast chicken

So for a learner, it is easiest to understand mrożone here simply as an adjective meaning frozen.

It could be understood, but mrożone warzywa is the more natural phrase for food sold as frozen vegetables.

Very roughly:

• mrożone = frozen, kept frozen, sold frozen
• zamrożone = frozen solid, made frozen

In everyday shopping language, mrożone warzywa is the normal expression you would see on packaging or in conversation.

Yes. Polish word order is more flexible than English, although some versions sound more neutral than others.

The original sentence is natural and neutral:

Kupuję mrożone warzywa, jeśli nie mam czasu iść na rynek po świeże.

You could also say:

Jeśli nie mam czasu iść na rynek po świeże, kupuję mrożone warzywa.

That puts more emphasis on the condition first: If I don’t have time...

Both are correct. The choice depends on emphasis and style.

It can mean both in different contexts.

• rynek can mean the market square in a town or city
• it can also refer to a marketplace, especially in everyday speech

In this sentence, because we have po świeże and we are talking about vegetables, the meaning is clearly the market / marketplace where you go to buy fresh produce.

So the context removes the ambiguity.