The difference between E and F and between B and C is a half step, or a single fret apart.  The rest of the notes are whole steps and separated by a fret (sharp or flat) between them.  These sharps or flats between whole steps are called “accidentals”.

We use a natural sign to cancel an accidental previously used:

Accidentals, including the natural sign, only apply to the bar in which they are used.

I made the statement back in lesson #1, “we say "G#" instead of "Ab" and "C#" instead of "Db", don't we.”