New Super Mario Bros. Ds how do you get to the

1. becasuse u need to eteir buy a green mushroom store or a red one or complete a casel like the small one or the big or go on to the next level

try the secret exit
At the end of the level is a bunch of wooden blocks with a green pipe underneath
them. There are two ways of getting here; either use the surrounding Bob-Ombs
to open it up or easier use a Mega Mushroom
in reserve to Ground Pound the way down to the pipe.

There is mushroom, mini mushroom, mega mushroom, blue shell, fire flower and invincible star

you have to go to world 7 and beat the boss as mini mario aka use a mini mushroom.

I used a giant mushroom to beat it.

Use a mini mushroom

You need to get the mini mushroom's in each castle and beat the boss of each castle while staying mini for both world 4 and 7 to be unlocked.

WHAT SECRET LEVELS DO YOU MEAN WORLD 4 AND 7
I KNOW THE SOLUTION JUST GO ON WORLD 2 THEN GET A TINY MUSHROOM THEN FIGHT THE BOSS IN THE SECOND CASTLE IN YOUR TINY FORM THE SAME AS WORLD 7

The mushroom houses at the start of each world (the point with an arrow at the start) differ determining on the time you get on a stage. If you get x44, x55, x66 on the timer, a Green Mushroom house appears, which is a 1up minigame.
BONUS TIP: In World 7-5, throw a Koopa shell in-between the two Bullet Bill cannons to get an extra life every four seconds

:)

When you get to the mid-level point, there are two rows of bricks and two jumping turtles. The top row question mark holds a mushroom to make you mid-size. Knock out the brick to the left of this. Then go down to the bottom row and the middle brick (directly below the above hole) holds a vine. Hit the brick to make the vine grow and climb up.

Once you are up in the clouds, jump on the mushroom. Be careful not to fall off. Ride the mushroom past the gold coin, keep going and there is a secret pipe. Jump up and go up the pipe. This will get you past world 7-6.