Đây không phải lọ thủy tinh mà là chai nhựa.

This is a very common Vietnamese pattern for correcting or contrasting identification:

• không phải X mà là Y = not X, but Y

So:

• Đây không phải lọ thủy tinh mà là chai nhựa.
• This is not a glass jar, but a plastic bottle.

Why phải?
Because phải here is part of the expression không phải, which often means is not, is not the case, or is not actually in identification sentences.

You usually cannot replace it with just không là in normal Vietnamese.
For this kind of sentence, không phải ... mà là ... is the natural structure.

Đây means this or this here.

In this sentence, đây points to something near the speaker:

• Đây = this
• kia = that over there

So Đây không phải... means This is not...

A beginner-friendly way to think of it:

• đây often works like this is
• đó/kia often work like that is

Vietnamese often leaves out a verb equivalent to English to be in simple identification sentences, especially when the structure already makes the meaning clear.

So:

• Đây là chai nhựa. = This is a plastic bottle.
• Đây không phải lọ thủy tinh. = This is not a glass jar.

In affirmative sentences, là is often used.
In negative identification, không phải already does the job, so you do not say Đây không là...

That is why the sentence is:

• Đây không phải lọ thủy tinh...

not

• Đây không là lọ thủy tinh...

Both are container words, but they are not the same.

• lọ = jar, small container, often wider and shorter
• chai = bottle, usually taller and narrower

So:

• lọ thủy tinh = glass jar
• chai nhựa = plastic bottle

Depending on context, English translations can vary a little, but the basic contrast is:

• lọ -> jar
• chai -> bottle

In Vietnamese, descriptive words often come after the noun.

So the order is:

• lọ thủy tinh = jar glass -> glass jar
• chai nhựa = bottle plastic -> plastic bottle

This is normal Vietnamese word order.
English usually puts the material first, but Vietnamese usually puts it after the noun.

More examples:

• bàn gỗ = wooden table
• cốc giấy = paper cup
• thìa bạc = silver spoon

They are originally nouns:

• thủy tinh = glass
• nhựa = plastic

But in combinations like this, they function a lot like material descriptors, similar to adjectives in English.

So in:

• lọ thủy tinh
• chai nhựa

the second word tells you what the object is made of.

This is very common in Vietnamese: a noun can modify another noun without needing a special adjective form.

mà là introduces the correction or the true identification after the first idea is rejected.

Pattern:

• không phải X mà là Y
• not X but Y

So in this sentence:

• first, lọ thủy tinh is rejected
• then, chai nhựa is given as the correct identification

You can think of mà là as roughly but rather / but instead is

Usually, no—not in this exact structure.

For direct correction, Vietnamese strongly prefers:

• không phải X mà là Y

not:

• không phải X nhưng là Y

Nhưng means but, but it is used more for general contrast between clauses, not this specific correction pattern.

So the natural sentence is:

• Đây không phải lọ thủy tinh mà là chai nhựa.

Good question. Vietnamese classifiers are common, but they are not always required.

You may hear:

• Đây không phải cái lọ thủy tinh mà là cái chai nhựa.

This is also grammatical.

Without cái, the sentence is still perfectly natural, especially in simple identification or labeling contexts.

Very roughly:

• without classifier: slightly leaner, more dictionary-like or neutral
• with classifier: sometimes more conversational, more specific to the object

So both can work, depending on style and context.

They can sound very natural in everyday speech, yes.

Compare:

• Đây không phải lọ thủy tinh mà là chai nhựa.
• Đây không phải cái lọ thủy tinh mà là cái chai nhựa.

Both are okay.

The version without cái can feel a bit more concise.
The version with cái can feel a bit more like spoken conversation about a specific object in front of you.

So this is often a matter of nuance, not strict right vs. wrong.

Yes. That is also correct.

It means:

• This is a plastic bottle, not a glass jar.

The difference is mainly focus:

• Đây không phải lọ thủy tinh mà là chai nhựa.
  Starts by correcting the wrong idea first.

• Đây là chai nhựa, không phải lọ thủy tinh.
  Starts by giving the correct identification first.

Both are natural, but the original pattern is especially useful when correcting someone.

In many everyday cases, yes, especially when talking about material.

• lọ thủy tinh = glass jar
• cốc thủy tinh = glass cup
• chai thủy tinh = glass bottle

You may also hear kiếng in some regional contexts, especially in the South, but thủy tinh is the standard word for the material glass in this kind of sentence.

Lọ is widely understood, but regional preferences exist.

Depending on region and object type, people might also use words like:

• hũ = jar
• chai = bottle
• bình = bottle/container/flask in some contexts

So lọ thủy tinh is correct and understandable, but another region might prefer a slightly different word for the same object, especially if the container shape is different.

A natural rhythm would group it like this:

• Đây / không phải / lọ thủy tinh / mà là / chai nhựa.

A few pronunciation notes:

• đây has a level-ish tone with a glide, roughly like day but not exactly
• phải has the hỏi tone
• lọ has the nặng tone, short and low with a stop-like ending
• thủy has the hỏi tone
• tinh is fairly straightforward
• mà is low-falling
• chai sounds a bit like English chai
• nhựa can be tricky because of nh

If you want to sound natural, the biggest thing is not individual words alone, but the contrast pattern:

• không phải ... mà là ...

That part should sound clearly corrective.

Because the structure already contains mà là in the correction part, and không phải handles the negative identification.

So Vietnamese does not need:

• Đây là không phải...

That would be wrong.

Instead, the sentence is built as:

• Đây
  • không phải X
    • mà là Y

This is a complete and natural structure by itself.

Yes, absolutely. That is one of its most common uses.

For example:

• Someone says it is a glass jar.
• You reply: Đây không phải lọ thủy tinh mà là chai nhựa.

So the sentence is not just describing an object—it is also correcting a mistaken assumption.

That is why the pattern không phải ... mà là ... is so useful in conversation.