How do you modulate to the subdominant?
For example, relative to the key of C major, the key of F major is the subdominant. Music which modulates (changes key) often modulates to the subdominant when the leading tone is lowered by half step to the subtonic (B to B♭ in the key of C).
How do you modulate the dominant?
This technique is a useful way to modulate to the dominant: a composer can end a phrase with a half cadence (on the dominant chord) and then simply begin the next phrase in the dominant key.