Learn Swedish: Glider den nya skidan fortare? - Does the new ski glide faster?

It comes from the base adverb fort (fast). To make the comparative form (meaning "more" or "-er"), regular Swedish adverbs simply add -are to the end. This perfectly matches the pattern for regular adverbs: fort becomes fortare.

Could I use snabbare instead of fortare here?

Yes, you could! Both fortare and snabbare mean "faster" and are often used interchangeably when talking about speed. Fort is strictly an adverb (used with verbs, like gliding), whereas snabb is an adjective that can also become an adverb.

Why does the sentence use both den and the definite ending on skidan?

This is a great reminder of the Swedish "double definite" rule. When you describe a specific noun using an adjective (nya), you must use the standalone article den (the) and keep the definite suffix on the noun itself (skidan). Literally, the sentence says "the new the ski".

Why is the adjective written as nya instead of just ny?