I believe Alison gave the best answer for 'thanks' = 'xie xie' (谢谢 as pronounced in 4th and in neutral tone), 'ni' (你 in 3rd tone). Good job too, KK/Jennifer! But if you add the "ni (你）to 'xie xie', it will become 4th tone.

谢谢~xie4 xie4 非常感谢thanks a lot (fei1 chang2 gan3 xie4)

the simple one that I know is "xie xie"

Three common ways to say thank you in Chinese: 1. 谢谢[xiè xie]， thank you 2. 谢谢你（您) [xiè xiè nǐ （nín )], thank you 3. 谢谢你的帮助 [xiè xiè nǐ de bāng zhù]， thank you for you help. If you also want to know how to response thank you in Chinese, check it out http://www.hanbridgemandarin.com/article/daily-chinese-learning-tips/how-to-say-thank-you-in-chinese/

xie xie=thanks