A question with 吧 indicates that the speaker is almost certain of the situation, but not 100% sure. Frequently, adverbs such as 一定 (yídìng: ‘defi nitely’) and 大概 (dàgài: ‘probably’) can be used in such a question, but 吧 alone can serve the function of indicating near certainty.It is acceptable to use an exclamation mark instead of a question mark after 吧.
Nǐ gōng zuò le yī zhěng tiān，(yī dìng)lèi le ba !
You worked all day; you must be tired, aren’t you?
Zhè běn shū shàng xiě zhe nǐ de míng zì，shì nǐ de ba ？
This book has your name written in it. It must be yours, isn’t it?
 Yip, P. C., Rimmington, D., Xiaoming, Z., & Henson, R. (2009). Basic Chinese: a grammar and workbook. Taylor & Francis.
 Teng, W. H. (2016). Yufa! A practical guide to Mandarin Chinese grammar. Taylor & Francis.