Learn Polish: Drewno jest lepsze niż plastik. - Wood is better than plastic.

The adjective has to match the gender of the noun it describes. Because drewno (wood) is a neuter noun (it ends in "-o"), the adjective takes the neuter ending -e, making it lepsze. If you were talking about a masculine noun, you would use lepszy.

Is lepsze an irregular word? What is the basic word for "good"?

Yes! Just like "good" changes to "better" in English, Polish has an irregular comparison here. The basic neuter word for "good" is dobre (from the masculine dobry). To say "better", the word completely changes to lepsze.

Why doesn't the word plastik change its ending after niż?

The word niż (than) doesn't force the noun after it to change its case. Instead, it works like a mirror: the two things you are comparing must be in the same case. Since the first word, drewno, is the subject of the sentence and is in the basic nominative case, plastik stays in the nominative case too.

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

Yes, you can! Both mean "than". However, if you use the preposition od, it forces the next word into the genitive case. So, you would have to say: Drewno jest lepsze od plastiku. Using niż is often easier because you don't have to change the ending of the second noun.