World 1 (Super Mario World 2: Yoshi's Island)
From the Super Mario Wiki
World 1 is the first world of the game Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3. The world contains eight normal levels and two secret levels. The world's theme is a grassy plain in spring.
 Levels of World 1
The following is a list of each level in World 1.
The second level is easy too. However, at the beginning, Yoshi must run quickly because Chomps will fall down and create holes in the ground. This level also introduces the transformation of Yoshi into Yoshi Helicopter.
The first fort is an easy level. At the beginning Yoshi must be careful because some walls can fall down and squash him when Yoshi runs past them. The level features the Burts, Podoboo and much more. The boss of the fort is Burt The Bashful, a giant Burt who jumps very high. Yoshi must throw eggs at him to defeat him.
This level is slightly harder than the previous four because it is an auto-scrolling level, so Yoshi must follow the path to get to the end. The level features the Donut Lifts as says the name, which fall if Yoshi stands on them for too long.
This is an easy level with many Shy Guys on stilts. This special kind of Shy Guy can't be swallowed by Yoshi when on their stilts. This level contains two dark cave areas. At the end, an 1-Up balloon will appear.
This is the first castle of the game. It's an easy level, however Yoshi must be careful with the spiked platforms and the rotating cylinders. The boss is Salvo the Slime, a giant Lemon Drop. Yoshi has to throw eggs at it to defeat it.
Available only in Super Mario Advance 3: Yoshi's Island. This is the first secret level in the game. As the name implies, Yoshi will have to traverse through cloud platforms and other types of airborne terrain most of the time.