暇 functions as both a noun (free time) and a na-adjective (not busy).
FORMS:
- Na-adjective: 暇な時 (when free)
- Noun: 暇がある (have free time)
- Adverbial: 暇に (in one's free time - rare)
COMMON EXPRESSIONS:
- 暇がある: to have free time
- 暇がない: to have no free time
- 暇をつぶす (暇つぶし): to kill time
- お暇する: to take one's leave (polite)
CONTRAST: