你要是碰见她，帮我跟她说声谢谢。

Nǐ yàoshi pèngjiàn tā, bāng wǒ gēn tā shuō shēng xièxie.

If you happen to see her, please help me say thank you to her.

上周我在公园碰见一个很久没见的朋友。

Shàngzhōu wǒ zài gōngyuán pèngjiàn yí gè hěn jiǔ méi jiàn de péngyou.

Last week I met a friend in the park whom I hadn't seen for a long time.

我没想到会在这里碰见你们全家。

Wǒ méi xiǎngdào huì zài zhèlǐ pèngjiàn nǐmen quánjiā.

I didn't expect to meet your whole family here.