Powinienem był kupić więcej jogurtu, bo dzieci zjadły go od razu.

Together, powinienem był means I should have.

• powinienem = I should / I ought to (said by a male speaker)
• był adds a past perspective

So Powinienem był kupić... means I should have bought..., not just I should buy....

This structure is very common in Polish for talking about an obligation or good idea that was not carried out in the past.

Because Polish usually drops subject pronouns when they are already clear from the verb form.

In powinienem, the ending already tells you:

• first person singular = I
• masculine speaker

So ja is unnecessary unless you want emphasis, contrast, or emotional stress:

• Ja powinienem był kupić więcej jogurtu = I should have bought more yogurt (with emphasis on I)

Yes.

A male speaker says:

• powinienem był

A female speaker says:

• powinnam była

So the full sentence said by a woman would be:

• Powinnam była kupić więcej jogurtu, bo dzieci zjadły go od razu.

This is something English speakers often notice because English does not mark the speaker’s gender in this way.

Because after powinienem / powinienem był, Polish uses an infinitive, just like English uses the base verb after should.

Compare:

• I should buy
• I should have bought

In Polish:

• powinienem kupić
• powinienem był kupić

So kupić stays in the infinitive. You do not use a past-tense form like kupiłem after powinienem był.

Because kupić is perfective, and here the speaker means one completed purchase.

The idea is:

• there was a need for a single action
• that action should have been completed
• it was not done

So kupić fits very well.

If you used kupować, it would sound more like:

• buying in general
• buying repeatedly
• focusing on the process rather than the completed result

In this sentence, the point is that one more completed purchase would have solved the problem.

Because więcej normally requires the genitive case.

So:

• jogurt → dictionary form
• jogurtu → genitive singular

That gives:

• więcej jogurtu = more yogurt

Since jogurt here is treated as a mass noun, the singular genitive is natural.

If you meant individual yogurts, cups, or containers, you could also hear:

• więcej jogurtów = more yogurts / more yogurt cups

Because dzieci takes the non-masculine-personal plural past ending -ły.

In Polish plural past tense:

• -li is used with masculine-personal groups
• -ły is used with non-masculine-personal groups

Even though dzieci refers to people, grammatically it patterns with the -ły type:

• dzieci zjadły
• dzieci przyszły
• dzieci poszły

This is normal Polish grammar.

Because zjadły is perfective and means the children ate it up / finished it.

That matches the context very well:

• there was yogurt
• the children consumed it completely
• it disappeared quickly

If you used jadły, it would describe the process of eating, not the completed result:

• dzieci jadły go od razu would sound less natural here unless you specifically wanted to focus on the action in progress

So zjadły is the better choice.

go means it and refers back to the yogurt.

So:

• dzieci zjadły go od razu = the children ate it right away

Polish often uses such object pronouns when the object has already been mentioned and is still important in the sentence.

Without go, the sentence could still be understood from context, but it would be less explicit.

Here go functions as the direct object of zjadły, so it is the object pronoun meaning it.

A useful thing to remember is this:

• jogurtu is genitive because of więcej
• go is used because the children ate it

So the pronoun does not have to copy the case of jogurtu from the earlier phrase.

Also, the form go looks the same in both the accusative and genitive for this pronoun, which can confuse learners. In this sentence, it is functioning as the direct object.

Od razu means:

• right away
• immediately
• at once

It is a very common everyday expression.

In this sentence, it means the children ate the yogurt as soon as they had the chance.

No. Polish word order is fairly flexible.

The version you have is natural and neutral:

• Powinienem był kupić więcej jogurtu, bo dzieci zjadły go od razu.

But other orders are also possible:

• Powinienem był kupić więcej jogurtu, bo dzieci od razu go zjadły.
• Więcej jogurtu powinienem był kupić, bo dzieci zjadły go od razu.

These versions change emphasis more than basic meaning.

One small point: short pronouns like go often avoid strong stress, so they usually stay in the middle of the sentence rather than being heavily highlighted.

Yes, but the meaning changes.

• Powinienem kupić więcej jogurtu usually means I should buy more yogurt or I ought to buy more yogurt
• Powinienem był kupić więcej jogurtu clearly means I should have bought more yogurt

So if you want the past meaning in clear, standard Polish, był is important.

Because bo is the normal everyday word for because.

So:

• bo = common, conversational, very natural in speech
• ponieważ = more formal or written

Both are correct, but bo sounds more natural in an ordinary sentence like this one.