The mood that Chinese modal particles express is a complicated system you will find when you learn Chinese in China . Every modal particle can cover the moods in a certain register. If several modal particles are collocated with each other, all kinds of complicated and rich mood can be expressed out.
The moods of modal particle “吧” are mainly in the register of being not tough, complying and uncertain.
“Being not tough” is often shown in function Chinese sentences of urging, suggesting, consulting and requesting from strong mood to weak. These function Chinese sentences show that the speaker gives the decision right to the listener from urging to requesting with the use of the modal particle “吧” which you need to know when you learn Chinese in China. The mood will be weakened as the listener’s decision right is strengthened. For example,
(1) 快走！——urging——the listener has no decision right, it is close to command or it is command.
快走吧！——urging or requesting——the listener has certain decision; the speaker’s will is not tough.
(2) 不行。——tough will and not negotiable
不行吧。——irresolute, uncertain; the listener has right to negotiate
(3) 这样可以。——not consulting——the speaker makes decisions
这样可以吧。——consulting mood is stronger——hoping to be accepted by the listener and the listener has decision right
(4) 你帮他一下。——making request and not negotiable
你帮他一下吧。——mood of requesting is stronger and the listener has right to negotiate
“吧”can also be used to express agreement which you need to know when you learn Chinese in China . This kind of expression is mainly in the register of complying, say complying the advice of the listener, which essentially means that the decision right is transferred to the listener, for example,
(5) A. 她最近心情不太好，咱们去看看她吧。——suggesting or consulting and the listener has the decision right
B. 好吧。——agreement and complying the suggestion of the listener
(6) A.根据大家的意见，我把方案修改了一下，您看看行不行？——asking the opinion of the listener
B. 行啊，就这样吧。——agreement and complying everyone’s opinion
When “吧”is used to express the mood of questioning, it is mainly in the register of uncertainty. For example,
(7)那个建议是老王提的吧？——known a little, but not sure
(8)明天不会下雪吧？——conjecturing according to the weather, but not sure
The basic function of preventing and prohibiting Chinese sentences is to express the speaker’s tough subjective will and decision, whose goal is to prevent and forbid. Therefore consulting, complying and requesting are not needed and the listener is supposed to obey and has no decision right. For example,
(9)别吵了，有话好好说。
(10) 不许用那种语气跟长辈说话。
(11) 禁止吸烟！
Hence, in Chinese, sentence patterns like “别……了”, “不许……”, “禁止……” are not likely to use the mood of “吧”. And the tougher the mood is, the more impossible “吧” will be used. Some Chinese learners don’t understand the mood of “吧” exactly and often use it in preventing and prohibiting sentences, which makes their expression of mood inappropriate. For example, “*你不要大声说话吧！”, “*你不许骗他吧。”
Of course, the mood that the speaker expresses and the certainty that the speaker has to objective truth can be not that consistent that you need to know when you learn Chinese in China . That is because that the communication strategy of dialogues is still working. For example, a speaker knows exactly that his boss has a meeting at 3, but he will choose the uncertain mood expression with “吧” on purpose——“主任，您3点有会吧？”——to remind or urge the listener, but won’t let his boss feel embarrassed.
Thus, Chinese model particle is one of the ways that form the subtle expression. To use it properly can make the expression vivid, mild and proper.