Learn Polish: To dziecko, które widziałem w parku. - This is the child that I saw in the park.

The relative pronoun must match the gender of the noun it describes. Since it refers back to dziecko (child), which is a neuter noun, we use the neuter form które.

Which case is które in this sentence?

It is in the accusative case because it acts as the direct object of the verb widziałem (I saw). For neuter nouns and pronouns, the accusative form looks exactly the same as the nominative form, which is why it stays które.

Does To dziecko mean 'This child' or 'This is the child'?

Here it means 'This is the child'. Polish often drops the verb jest (is) in simple identification sentences, so To jest dziecko becomes just To dziecko.

How can we tell the gender of the person speaking?

We look at the verb widziałem (I saw). The -em ending tells us the speaker is male. If a woman were speaking, she would say widziałam.

Why is it w parku and not w park?