Learn Polish: Butelka, która tam stoi, jest pełna. - The bottle which is standing there is full.

The relative pronoun który changes its ending to match the gender and number of the noun it describes. Because it refers back to butelka (bottle), which is a feminine singular noun, you must use the feminine singular form która.

Do I always need to put commas around która tam stoi?

Yes! In Polish, you must always place a comma before relative pronouns like która. Because this extra description interrupts the main sentence (Butelka... jest pełna), you also need a second comma to close the description off before finishing your thought.

Why does the Polish sentence say the bottle 'stands' (stoi) instead of just saying it 'is' there?

Polish prefers to be specific about how an object is positioned in space. Because a typical bottle rests upright on its base, it 'stands' (stoi). If the bottle were empty and knocked over on its side, you would say it 'lies' (leży) there instead.

Why does the sentence end with pełna and not pełny?

Pełna means 'full'. It is an adjective describing the butelka. Since butelka is a feminine noun in the nominative case, the adjective must also take the feminine nominative ending -a.