Learn Polish: Dzisiaj nie ma słońca, a woda jest zimna. - Today there is no sun, and the water is cold.

The phrase nie ma (there is no) always forces the noun that follows it into the Genitive case. The dictionary form for 'sun' is słońce. Because it is a neuter noun ending in -e, it changes to -a in the Genitive, becoming słońca.

Since we used nie ma, shouldn't woda also be in the Genitive case?

No, because the grammar rule for nie ma only applies to the thing that is missing! The second half of the sentence (woda jest zimna) is a completely separate thought with its own verb (jest). Because it's a simple statement saying what the water is, woda stays in the basic dictionary form (Nominative case).

Why does the sentence use a for 'and' instead of i?

In Polish, i is used simply to add similar things together. The word a is a contrastive 'and'. It is used to link two different or slightly contrasting situations, often translating to 'and', 'but', or 'while'. Here, it links two distinct statements: the lack of sun and the state of the water.

Why does the word for cold end in -a (zimna)?