There are many use-cases of 吧. In these 吧 should be placed at the end of the sentence.
First the 吧 is used when making polite suggestions, requests or orders. Its presence will soften the tone of such a request or suggestion.
- 走吧 (zou3 ba): let's go
- 帮帮他吧! (bang1 bang1 ta1 ba): help him!
- 我们赶快去医院看她吧 (wo3 men gan3 kuai4 qu4 yi1 yuan4 kan4 ta1 ba): let's go quickly to the hospital to see her
- 饭菜凉了，快一点儿吃吧 (fan4 cai4 liang2 le, kuai4 yi4 dianr3 chi1 ba): the food has become cold, please start eating
- 如果你在英语方面有问题，就问我吧 (ru2 guo3 ni3 zai4 ying1 yu3 fang1 mian4 you3 wen4 ti2, jiu4 wen4 wo3 ba): if you have questions related to
English, ask them
- 你们快回家去吧 (ni3 men kuai4 hui2 jia1 qu4 ba): you'd beter go back home right away
- 你就放心吧 (ni3 jiu4 fang4 xin1 ba): you can count on me
- 咱们喝点什么吧 (za2 men he1 dian3 shen2 me ba): let us drink something
Secondly it is used to express approval. Some examples:
- 好吧 (hao3 ba): OK
- 好吧，听你的 (hao3 ba, ting1 ni3 de): all right; whatever you say/we'll do it your way
Thirdly it is used to ask a question (involving a conjecture or to indicate doubt without seeking an answer or to soften the question). Some examples:
- 你不会不知道吧？ (ni3 bu2 hui4 bu4 zhi1 dao4 ba): How can't you know (you have to know it already)
- 你有一点儿累了吧？ (ni3 you3 yi4 dianr3 lei4 le ba?): You are a bit tired, aren't you? (This is used for example you can see from the person's face that is tired so it's more a rhetorical question)
- 他到三十岁了吧？ (ta1 dao4 san1 shi2 sui4 le ba?): has he reached 30?
- 这是我的吧？ (zhe4 shi wo3 de ba ba): This is mine, right? (instead of 这是我的）
Fourthly when you are not completely certain, you want to indicate uncertainty or supposition. It will soften the tone. Some examples:
- 这个汉字不对吧 (zhe4 ge han4 zi bu2 dui4 ba): This Chinese character is wrong, I guess. (Compare this sentence to 这个汉字不对. The tone of the latter is much stronger. It's an authorative statement, while the former expresses doubt).
- 大概是前天吧 (da4 gai4 shi4 qian2 tian1 ba): It most have been the day before yesterday