búyào bǎ zhège fàng zài zuǐ lǐ.

This is because of a common tone change rule for 不.

• 不 is normally fourth tone: bù
• But when it comes before another fourth-tone syllable, it changes to second tone: bú

Since 要 yào is fourth tone, 不要 is pronounced búyào.

This change affects pronunciation, not meaning.

Here, 不要 means don’t.

In Chinese, 不要 + verb is a very common way to tell someone not to do something:

• 不要说话 = Don’t talk.
• 不要动 = Don’t move.
• 不要把这个放在嘴里 = Don’t put this in your mouth.

In other contexts, 不要 can also mean do not want or do not need:

• 我不要。 = I don’t want it.

So the meaning depends on context. In this sentence, it is clearly a negative command.

把 is used to bring the object forward and show what happens to it.

The basic idea is:

• 把 + object + verb + result/location/etc.

So in this sentence:

• 把这个放在嘴里
• literally: take this and put it in the mouth

The speaker is focusing on what is being handled: 这个.

This pattern is very common when:

• the object is specific
• the action affects it directly
• the sentence says where it goes, what happens to it, or what result comes from the action

Without 把, the sentence is still possible in some situations, but 把 makes the structure especially natural for this kind of action.

Chinese and English organize this kind of sentence differently.

In English, you say:

• Don’t put this in your mouth.

In Chinese with 把, the order is:

• 不要 + 把 + object + verb + location
• 不要 把 这个 放 在 嘴里

So the object 这个 comes before the main verb 放.

That is one of the main features of the 把 construction: it moves the object before the verb.

In this sentence, 放在 works as put in/on/at.

You can think of it as:

• 放 = to put, to place
• 在 + location = at/in/on a place

So:

• 放在嘴里 = put it in the mouth

The 在 here introduces the location of the action or the result of the placement.

Similar examples:

• 放在桌子上 = put it on the table
• 放在包里 = put it in the bag

So 在 is not exactly part of the verb by itself, but 放在 + place is a very common combination.

里 means inside.

So:

• 嘴 = mouth
• 嘴里 = inside the mouth / in the mouth

Because the sentence means putting something into the mouth, 里 makes that idea clear.

You will often see this pattern with body parts and places:

• 手里 = in the hand
• 包里 = in the bag
• 房间里 = in the room

Saying just 嘴 here would sound incomplete or unnatural.

A native speaker would usually prefer 不要把这个放在嘴里.

Your version, 不要放这个在嘴里, is understandable, but it sounds less natural in standard Mandarin.

Why?

Because when the object is specific (这个) and the sentence tells where it is being put, Chinese often prefers the 把 construction:

• 把这个放在嘴里
• not usually 放这个在嘴里

A more natural alternative without 把 would be something like:

• 不要把这个放嘴里
• 这个不要放在嘴里

But the original sentence is the safest and most standard form.

Yes. 别 is very common in spoken Chinese for don’t.

So you could also say:

• 别把这个放在嘴里。

This means the same thing.

A rough comparison:

• 不要 can sound a little fuller or slightly more formal/explicit
• 别 is shorter and very common in everyday speech

Both are correct here.

No noun is required here. 这个 can mean this one or simply this.

So:

• 这个 = this / this thing

Chinese often uses demonstratives like this without adding a noun when the object is obvious from context.

For example:

• someone is holding a small object
• a parent says 不要把这个放在嘴里
• the listener already knows what 这个 refers to

So there is no missing word. The sentence is complete.

In careful pinyin, you may see zhège, but in natural speech the second syllable is often light or neutral:

• zhè ge

That means the second part may sound weaker and shorter than a full fourth tone.

Learners will hear both kinds of notation, but in real speech, a lighter second syllable is very common.

Yes, absolutely. It sounds very natural as a warning, especially to a child.

For example, if a child picks up a small object, an adult might say:

• 不要把这个放在嘴里。
• 别把这个放在嘴里。

Both are normal and natural.

In fast everyday speech, people might also shorten it a little:

• 别放嘴里。 = Don’t put it in your mouth.

But the full sentence is clear, standard, and very useful.

Yes. The subject is understood from context.

Chinese often leaves out subjects when they are obvious. In an instruction or warning like this, the listener is naturally understood as you.

So the sentence really means:

• You, don’t put this in your mouth.

But Chinese normally does not need to say 你 unless there is special emphasis:

• 你不要把这个放在嘴里。

Adding 你 is possible, but not necessary.

In Chinese, possession is often left unstated when it is obvious.

So 在嘴里 literally means in the mouth, but in context it naturally means in your mouth.

Chinese does this a lot with body parts:

• 抬手 = raise your hand
• 闭上眼睛 = close your eyes
• 张开嘴 = open your mouth

If needed, you can say 在你的嘴里, but here that would usually sound unnecessary.