In linguistics, modal particles are always uninflected words, and are a type of grammatical particle. Mandarin Chinese depends highly on the use of modal particles. Changing the pitch of the sentence or phrase would make the meaning of the sentence different.

啊(ā) is placed at the end of a sentence to express admiration or affirmation, for example:

duō hǎo de dì fɑnɡ ā
多 好 的 地 方 啊！
What a place!

nǐ kě yào xiǎo xīn ā
你 可 要 小 心 啊！
Do be careful!

Or to indicate a question, for example:

nǐ men nǎ tiān qù lǚ xínɡ ā
你 们 哪 天 去 旅 行 啊？
When are you going to start on your journey?

啊 can also be placed in the sentence to pause for a second and draw attention to the following sentences, for example:

nǐ ā lǎo zhè yànɡ xià qu kě bù xínɡ
你 啊 ，老 这 样 下 去可 不 行！
Look! You can’t go on like this.

Or placed after a repetitive verb to indicate that it is a very long process:

tā men zǒu ā zǒu ā yì zhí zǒu dào hé biān
他 们 走 啊，走 啊，一 直走 到 河 边。
They went on and on till they came to a river.

呀(yɑ): ah; oh

yɑ xià xuě le
呀，下 雪 了！
Oh, it’s snowing!

shuí yɑ
谁 呀？
Who is it?

zhēn shānɡ xīn yɑ
真 伤 心 呀！
Woe is me!

ò
哦: Oh; ah; I see

ò wǒ dǒnɡ le
哦，我 懂 了。
Oh! I see. Oh! Now I understand.

ó zhēn de
哦？真 的？
Oh, you do?

mɑ
吗: placed at the end of a sentence to indicate a question

shì mɑ
是 吗？
That so?

ké yǐ mɑ
可 以 吗？
Would you?

tánɡ cù yú hǎo chī mɑ
糖 醋 鱼 好 吃 吗？
How do you like the sweet and sour fish?

mɑ
嘛: to indicate it is obvious

zhè yě bù nénɡ ɡuài tā tóu yì huí mɑ
这 也 不 能 怪 他，头 一回 嘛。
He’s not to blame. After all, it was the first time he’d done it.

Or, placed at the end of a sentence to draw attention to the following sentences:

nǐ mɑ jiù bú yònɡ qīn zì qù le
你 嘛，就 不 用 亲 自 去了。
As for you, I don’t think you have to go in person.

(Source: 米来汉语)