Sharps and flats on Mario Paint Composer?

Answer 1

Holding the shift key as you place a note will make the note sharp.

Holding the Alt or control Key as you place a note will make it flat.

Existing notes cannot be made sharp or flat; they must be replaced if you want them to be sharp or flat.

Answer 2

Although I'm pretty sure there is no half note, I know sharps and flats.

Sharps: Hold down the "Shift" key then place the note while holding the shift key down.

Flats: Hold the Ctrl or Alt key then place the note while holding the Ctrl or Alt key down.

(and if this helps... the car is kind of like a half note.)