Appropriate：是形容詞指「恰當」、「適當」；帶有「合理性」的意思。
- Judy's outfit isn't appropriate for school.
- It's not appropriate to wear bright clothes to funerals.
Suitable：是形容詞，跟「合適」意思相近；帶有在「功能」上合適的意思。
- Your jacket isn't suitable for this weather.
- This water-proof watch is suitable for swimming.
Properly：是副詞，形容詞是Proper，有「正確」、「合宜」的意思，常做「恰當」、「適當」解。
- It's not proper to talk loudly on the bus.
- Please make sure the windows are sealed properly. Otherwise, the rain will seep through the cracks.
Applicable：是「適用」的意思，通常跟「條件」、「資格」有關。
- You have to declare if you bring more than US$10,000 into Taiwan. This rule is applicable to everyone entering Taiwan.
- Many of the laws established hundreds of years ago are no longer applicable in today's world.
Opportune
Opportune是形容詞，跟Suitable的意思相近，唯獨Opportune通常是用來形容「時間上」的合適。
- I'll find an opportune time to discuss my resignation with my boss.
Befitting：是形容詞，意思相近Suitable和Appropriate。很多情況下可以互用，只是Befitting比較正式，因此較不常見。
- The company offered Mr. Wang a position befitting his experience.
- This policeman died trying to save a drowning boy. He should have a memorial service befitting a hero.
Felicitous：是一個形容詞，它的常用度比Befitting更低，在日常對話中很少人會用這個英文單字。Felicitous是用來表示既恰當、適合又讓人感覺愉快的。
- Jane has chosen some felicitous songs for tonight’s dinner party.
- The president made a felicitous speech at the beginning of the meeting.
