Grammar Lesson 12:
Multiple Third Tones
When a character using a third tone is followed by another character using a third tone, the first character changes to a second tone:
你好 (Hello) (ni3 hao3) changes to (ni2 hao3)
很好 (very good) (hen3 hao3) changes to (hen2 hao3)
Please be aware that in a dictionary it will ignore the tone change and still show "ni3 hao3" and "hen3 hao3" to indicate the original tone of the character. So it's important to know the rule to pronounce the character correctly.
Sometimes a small dot is used under the letter to indicate a tone change. This is not part of standard pinyin though and isn't very common. We do not indicate tone changes in our dictionary in that way.