Scratch

I am not going to show u bricks, instead tell you the theroy (in detail) so you can do it ure self and learn.
Basicly you want 2 variables, last good x and y and a if/else statment. If your charecter sprite is touching your ground sprite/colour then the the character's X and Y should be set to the variables, last good x and y. in the else, you are going to set your last good x/y to your current x/y. This sould provide you with perfect movment, up/down/left and right.
hope uve lernt!

In the character

<forever>
<if> <touching color[ ground
<change y by( positive #
[/blocks]

Keep the ground the same color.
If the ground is multiple colors, make it a sprite, and do
<touching[ ground sprite
[/blocks]instead of
<touching color[ ground color[/blocks]

Here is what you do, these are the scripts

<forever>
<if><touching[ ground
<change y by( 1 ) or two, it doesn't matter that much

now make a new script:

<forever>
<if> <<  <not>  >><touching[ ground
<change y by( -1 )


These two scripts make the sprite fall if not touching the ground and stay on if he is touching the ground. The first is for staying on and the second is for falling