方便：convenient

解析：convenient 指对某人来说很方便的，能省时省力省事的。

例句：Will it be convenient for you to come this afternoon? 翻译：你今天下午来方便吗？

例句：You'll find it very convenient to pay by credit card. 翻译：你会发现用信用卡付款非常便利。

例句：She will try to arrange a mutually convenient time and place for an interview. 翻译：她会尽力安排一个双方都方便的时间和地点进行面试。

合适：appropriate

解析：appropriate 指适合某一时间、情况、目的等的，常后接介词 for/to。

例句：It is appropriate to let him do the job. 翻译：让他来做这份工作是合适的。

例句：The situation is not appropriate for an attack. 翻译：形势不适合进攻。

例句：Dress neatly and attractively in an outfit appropriate to the job. 翻译：着装要整洁美观，适合于这份工作。

恰当的：proper

解析：proper 指正确而恰当的，侧重于符合某种标准或习惯，用于此义时只作定语。

例句：Make sure that everything is in its proper place. 翻译：务必把每样东西都放在恰当的位置。

例句：She wore proper clothes for the party. 翻译：她穿着适合聚会的服装。