From Lesson 10, a major scale can be represented by Roman numerals as such:
I represents the first note of the scale, called the root (or key) note, e.g., in the C major scale C is the root note. In the G major scale, G is the root note, i.e.:
By definition, for this major scale to have the correct sound, the 3rd-4th notes and 7th- 8th notes should be separated by only one semitone. F# must therefore be used to comply with the correct sequence, i.e., a tone between the 6th-7th notes and a semitone between the 7th and 8th notes.
Play up and down this G scale slowly, listening for the major scale sound (Do, Re, Mi etc.)