Learn Polish: Srebro jest tańsze niż złoto. - Silver is cheaper than gold.

It has to match the gender of the noun it describes, which is srebro (silver). Since srebro is a neuter noun, we use the neuter comparative form tańsze. If we were talking about a masculine noun like telefon, it would be tańszy.

The dictionary word for "cheap" is tani. How did it become tańsze?

When forming the comparative for adjectives ending in -ni, the n softens into ń before adding the comparative endings. That is why tani drops the i and takes the soft ń, giving us the neuter comparative ending -sze.

What case is the word złoto in after niż?

It is in the nominative case (the basic dictionary form). When you use niż (than) to compare two things, the second noun takes the exact same case as the first noun. Since srebro is in the nominative case, złoto must be as well.

Can I use od instead of niż to make this comparison?

Yes, you can! If you use od, the second noun must change to the genitive case. So, you could also say Srebro jest tańsze od złota. Both versions mean exactly the same thing and are used all the time in Polish.