Learn Polish: Lato jest cieplejsze niż wiosna. - Summer is warmer than spring.

The word niż (than) does not force the noun after it into a special case like a preposition would. When you use niż, the second thing you are comparing just copies the case of the first thing. Since lato is in the basic Nominative case as the subject of the sentence, wiosna stays in the Nominative case too.

Are there other ways to say 'than' besides using niż?

Yes! You can also use od (from) to make comparisons, but it forces the next word into the Genitive case. For example, you could say Lato jest cieplejsze od wiosny. Using niż is often easier for beginners because you can just leave the second noun in its basic Nominative form.

The dictionary word for 'warm' is ciepły. Why does it end in -e here?

Even though it is in the comparative form ('warmer'), it still acts like an adjective and must match the gender of the noun it describes. Since lato (summer) is a neuter noun, the comparative adjective takes the neuter ending -e, giving us cieplejsze. If it described a masculine noun, it would end in -y (cieplejszy).

How exactly does ciepły change into the comparative form cieplejsze?