What is neutral tone?
In Mandarin Chinese, unstressed syllables are often pronounced with a neutral tone, which is sometimes considered as a lack of tone. In connected speech, the neutral tone takes very little time to pronounce and does not carry a distinct tonal feature of its own. Instead, it takes on some aspects of the preceding tone, making the overall pronunciation smoother and more natural.
When following different tones, the pitch level of a neutral tone can differ. For example:
If the preceding tone is the first tone (high level tone): The neutral tone will often have a low pitch.
If the preceding tone is the second tone (rising tone): The neutral tone will often have a mid-to-low pitch.
If the preceding tone is the third tone (half third tone actually), the pitch level is around middle high pitch.
If the preceding tone is the fourth tone (falling tone): The neutral tone will often have a low pitch.
Pitch diagram of the neutral tone following the four tones in Chinese:

How do we know if a syllable is neutralized?
Whether a syllable is neutralized or not, one can’t tell from the Pinyin. In our textbook, we will either put a dot before a neutralized syllable or simply indicate it without a tone mark.
péng.yǒu (n. friend)
1st tone + neutral tone:
māma (n. mom):
gēge (n. elder brother):
shūshu (n. uncle):
xiānsheng (n. husband):
2nd tone + neutral tone:
háizi (n. child, children):
yéye (n. grandfather):
xuésheng (n. student, students)
huíqu (vp. go back, return)
3rd tone + neutral tone:
lǎolao (n. grandmother, mother’s mom):
nǎinai (n. grandmother, father’s mom):
jiějie (n. elder sister):
mǔqin (n. mother, formal saying):
4th tone + neutral tone:
tàitai (n. wife):
dìdi (n. younger brother):
mèimei (n. younger sister):
bàba (n. dad)