We know that in the Chinese pinyin system, there are not only the four tones, but there’s also a special tone we call the neutral tone, also called “toneless.” Actually, the neutral tone is not a fifth tone; it’s just a kind of change from the fourth tone. It is a soft and short one, which is used a lot in spoken Chinese. Now, let’s talk about it.
The neutral tone is divided into two groups: the regular neutral tone and the irregular neutral tone.
The Regular Neutral Tone
For the regular one, words or phrases should be read as a neutral tone.
1. Some affixation words, for example: 子, 头, 么, 儿, 巴, 里 are read as zi, tou, me, er, ba, li. Those words are non-referential, meaning an abstract indication, so they are always read with a neutral tone. For example: 裙子 (qún zi) skirt, 枕头 (zhěn tou) pillow, 什么 (shén me) what, 花儿 (huā er) flower, 结巴 (jiē ba) stammer, 稀里糊涂 (xī li hú tú) muddleheaded.
2. Some auxiliary words, for example: 得, 的, 了, 地, 着, 过 are read as de, de, le, di, zhe, guo. Examples as are follows: 好得很 (hǎo de hěn) very good, 我的 (wǒ de) my, 行了(xíng le) it worked, 小心地 (xiǎo xīn di) carefully, 看着 (kàn zhe) watching, 说过 (shuō guo) said. There’s another word 们 to be read as men, which makes the word plural. It can be used following nouns and pronouns. For example, 我们 (wǒ men) we and 动物们 (dòng wù men) the animals.
3. Some modal words 吧, 呢, 吗, 啊 are read as ba, ne, ma, a. For example: 好吧(hǎo ba) alright, 你呢 (nǐ ne) how about you, 好吗 (hǎo ma) shall we, 我走了啊 (wǒ zǒu le a) I'm going now.
4. Some words or phrases that are used before a verb or adjective show a trend. For example: 来 and 去 as in 爬上来 and 跑出去 which means (pá shàng lai) climb up and（pǎo chū qu) run out. And, some verbs show the result. For example: 打开(dǎ kai) open, 关上（guān shang）close, 看见（kàn jian）see.
5. Some words or phrases used before a noun or pronoun to show a direction, such as 上, 下, 里, 外, 边, 面. They are read as shang, xia, li, wai, bian, mian. For example: 黑板上 (hēi bǎnshang) on the blackboard, 地下(dì xia) underground, 宿舍里 (sù shè li) in the dormitory, 窗外(chuāng wai ) out of the window, 那边 (nà bian) there, 前面 (qián mian) ahead.
6. Some reduplicated nouns and verbs, such as 星星 (xīng xing) star, 爸爸（bà ba）father, 计划计划 (jì hua jì hua) make a plan, 学习学习（xué xi xué xi) study. Please take note of the 一 and 不 between duplicated words, for example: 看一看 (kàn yi kàn) have a look, 好不好 (hǎo bu hǎo) good or not.
7. Some measure words such as 个. For example: 这个 (zhè ge) this, 哪个 (nǎ ge）which.
The Irregular Neutral Tone
明白(míng bai) understand, 姑娘 (gū niang) girl, 耳朵 (ěr duo) ear; 暖和 (nuǎn huo) warm; 漂（piào liang) beautiful, 功夫（gōng fu）kung fu, 便宜（pián yi）cheap, 裁缝（cái feng）tailor, 喜欢 (xǐ huan) like, etc. For these, you will just have to memorize the use of the neutral tone.
I hope these examples are helpful, but of course the real way to study to the neutral toneis to just start listening for the patterns I described above, getting used to them, and then using them yourself!
Edited by Ilene Springer