吧 (ba) is a small word at the end of a sentence. It turns a sentence into a suggestion ("let's..."), makes a request sound softer, or shows that you are fairly sure of something and want the other person to agree. It is an HSK 1 grammar point and is said with a light, neutral tone.
Put 吧 at the end of a sentence with 我们 to suggest doing something together. 我们去吃饭 is a plain statement ("We're going to eat"); 我们去吃饭吧 is a suggestion ("Let's go and eat").
To suggest not doing something, put 不 before the verb, often with 了 to show a change of plan.
With 你, 吧 turns an order into a friendly suggestion or invitation. 你坐 sounds like a command; 你坐吧 sounds like "Please, have a seat". It is very common when offering something or giving advice.
吧 at the end of a question shows that you think you already know the answer and expect the other person to agree. It works like "..., right?", "..., aren't you?" or "I suppose..." in English.
In a statement, 吧 can also make a guess sound less certain, like "probably".
好吧 means "OK" or "All right then". It agrees to something, often after a little hesitation, so it can sound less keen than a plain 好.
Both 吗 and 吧 can end a yes/no question, but they are not the same. Use 吗 when you really don't know the answer. Use 吧 only when you think the answer is yes and are checking. Never put 吗 and 吧 together.
Also, don't add 吧 to a question that already has a question word such as 什么 or 哪儿. 你想吃什么吧？ is wrong; say 你想吃什么？. And remember that 吧 (ba) has a neutral tone, unlike 八 (bā) or 把 (bǎ).
Practise this grammar point on paper: 20 exercises using only HSK 1 words, with an answer key on a separate page. Free to print and copy for your class.
Every example on this page uses only words up to HSK 1. Paste your own sentences into the level checker to see which words, if any, are above your level.