Polite expressions are essential in daily Chinese. 谢谢 (xièxie) means thank you and the polite reply is 不客气 (bú kèqi), which means you are welcome. When someone apologizes with 对不起 (duìbuqǐ), you can respond with 没关系 (méi guānxi), meaning no problem or it doesn't matter. The phrase 请问 (qǐngwèn) means excuse me or may I ask, and it is used to politely get attention before asking a question, such as 请问洗手间在哪里 (qǐngwèn xǐshǒujiān zài nǎlǐ).
Mandarin verbs are not conjugated for tense. Instead, time words and particles indicate when something happens. The particle 了 (le) indicates a completed action or a change of state, as in 我已经吃了 (wǒ yǐjīng chī le, I have already eaten). The adverb 已经 (yǐjīng) emphasizes that the action is already complete. The particle 过 (guò) marks past experience, as in 我去过中国 (wǒ qù guò Zhōngguó, I have been to China). The progressive aspect is shown with 在 (zài), as in 我在看书 (wǒ zài kàn shū, I am reading a book) or 她正在做作业 (tā zhèng zài zuò zuò yè, she is doing homework right now), where 正在 adds emphasis.
To express future plans, use 要 (yào) or 想 (xiǎng), as in 我明天要去机场 (wǒ míngtiān yào qù jīchǎng, I am going to the airport tomorrow) or 我想去上海 (wǒ xiǎng qù Shànghǎi, I want to go to Shanghai). The modal particle 吧 (ba) softens a sentence into a suggestion or a guess: 我们走吧 (wǒmen zǒu ba) means let's go, while 他是老师吧 (tā shì lǎoshī ba) means he is probably a teacher. The question particle 吗 (ma) turns a statement into a yes/no question, as in 你想喝茶吗 (nǐ xiǎng hē chá ma). To ask how or why, use 怎么 (zěnme), as in 怎么去 (zěnme qù, how to go) or 你怎么来了 (nǐ zěnme lái le, why did you come). To express a small quantity, use 一点儿 (yìdiǎnr) or 一些 (yìxiē).