Learn Polish: Ta pogoda jest bardzo zła. - This weather is very bad.

Pogoda (weather) is a feminine noun, which you can usually tell because it ends in -a. The word for 'this' has to match the gender of the noun it points to, so we use the feminine form ta.

Why does the adjective zła end in -a?

Adjectives must also match the gender of the noun they describe. The dictionary form of the word 'bad' is the masculine zły. Because pogoda is feminine, we swap the ending to -a to make it zła.

This lesson is about using the Genitive case for absence. Why isn't the Genitive used in this sentence?

We use the Genitive case when we are talking about the lack or absence of something, usually with nie ma (there is no). In this sentence, we are simply describing something that already exists (the weather), so we use the basic Nominative case for both the subject (ta pogoda) and its describing adjective (zła).

If I just want to say 'The weather is very bad,' do I still need to use ta?

No, you can just leave it out and say Pogoda jest bardzo zła. Polish doesn't have words for 'a' or 'the', so pogoda by itself translates perfectly to 'the weather'. We only use ta when we specifically want to emphasize 'this' particular weather.