Rules in Mandarin Tones

Tone Rule 1: Neutral Tone
The neutral tone is not mapped on the tone chart because it differs from the other four tones in that it does not have a defined pitch contour. The neutral tone is pronounced quickly and lightly without regard to pitch. Syllables with a neutral tone have no tone mark (but are sometimes marked with a "5" or a "0" after the syllable).
This tone is usually very easy to pick up. Note that aside from grammatical particles, single syllable words cannot have a neutral tone.
Try listening to the following neutral tone syllables:

Syllable

Play

Syllable

Play

ma

yǐzi

le

hóuzi

bózi

 

 

Tone Rule 2: 3-3 to 2-3
When there are two third tones in a row, the first one becomes second tone. This rule is always followed, automatically, even though it will not be reflected in the pinyin.
Try listening to the following examples:

Originally

After applying the rule

Play

你好(nǐ+ hǎo)

你好(ní hǎo)

很好(hěn + hǎo)

很好(hén hǎo)

好懂(hǎo + dǒng)

好懂(háo dǒng)

There is also a podcast about this tone rule: Two Third Tones.

Tone Rule 3: 不
When the word 不 (bù) precedes a fourth tone, 不 changes to second tone (bú). This rule is always followed, automatically, even though it will not be reflected in the pinyin.
Try listening to the following examples:

Originally

After applying the rule

Play

不对(bù + duì)

不对(bú duì)

不去(bù + qù)

不去(bú qù)

不错(bù + cuò)

不错(bú cuò)

Tone Rule 4: 一

The character 一 (yī), meaning "one," is first tone when alone, second tone when followed by a fourth tone, and fourth tone when followed by any other tone. This rule is always followed, automatically, even though it will not be reflected in the pinyin.
Try listening to the following examples:

Originally

After applying the rule

Play

一个(yī + gè)

一个(yí gè)

一次(yī + cì)

一次(yí cì)

一半(yī + bàn)

一半(yí bàn)

一般(yī + bān)

一般(yì bān)

一毛(yī + máo)

一毛(yì máo)

一百(yī + bǎi)

一百(yì bǎi)