frank、open、plain、blunt、honest、sincere与straightforward这些形容词都有“坦率、诚实”之意。
frank : 强调毫无保留地畅所欲言，不受任何约束。
- a full and frank discussion
全面而坦诚的讨论 - There followed a frank exchange of views.
接下来大家坦诚地交换了意见。
open : 指不隐瞒自己的秘密，愿意向他人表露自己或公开暴露自己的缺点。
- There has been open hostility between them ever since they had that argument last summer.
自从去年夏天他们发生了那场争吵后，两人就一直处于公开的敌对状态。
plain : 指直言不讳，毫不做作或故弄玄虚。
- It's quite plain that they don't want to speak to us.
很明显，他们不想和我们说话。 - The reason is perfectly plain.
原因很清楚。 - I made it quite plain (that) (= explained clearly that) I wasn't interested.
我明白地表示我不感兴趣。
blunt : 指讲话坦白直率、不客气，但隐含不得体、不顾及他人感情等意味。
- I'll be blunt - that last piece of work you did was terrible.
我就直说了——你创作的最后一件作品糟糕透了。
（拓展学习：小词详解 | blunt）
honest : 指遵守正直等道德准则。
- She's completely honest.
她很诚实。 - I'd like you to give me an honest answer/your honest opinion.
我希望你给我一个诚实的回答／坦诚的意见。 - He had an honest face (= he looked like he could be trusted).
他看上去忠厚可信。 - To be honest (with you), I don't think it will be possible.
（跟你）说老实话，我认为那根本不可能。
sincere : 侧重指一个人出自内心地对某人某事表示一种诚意。
- a sincere apology
真诚的道歉 - He seems so sincere.
他看上去很真诚。
straightforward : 多用于答复、叙述等场合。指直截了当，不回避，或故弄玄虚。
- Roz is straightforward and lets you know what she's thinking.
罗兹很坦诚，会告诉你她的想法。
