Chỉ khi hướng dẫn viên giải thích, chú thích mới rõ.

It is a very common Vietnamese pattern meaning Only when ..., then ... or Not until ..., ....

So in this sentence:

• Chỉ khi hướng dẫn viên giải thích = only when the tour guide explains
• chú thích mới rõ = the captions/notes are clear only then

The pattern emphasizes that the second clause becomes true only under that condition.

Here, mới does not mean new. It marks the idea of only then, not until then, or only under that condition.

In the pattern chỉ khi ..., ... mới ..., mới is what makes the main clause sound complete and emphatic. Without it, the sentence would lose that strong only when meaning and sound less natural in this structure.

Yes, mới can mean new in other sentences, but not here.

In this sentence, mới is an adverb with a meaning closer to:

• only then
• not until then
• only in that case

So you should not understand chú thích mới rõ as new clear captions. It means the captions are clear only then.

Here, chú thích is a noun. It means something like:

• caption
• annotation
• note
• explanatory label

It is the subject of the second clause: chú thích mới rõ.

Although chú thích can also refer to the act of annotating in some contexts, in this sentence it is clearly a noun.

Because Vietnamese often uses adjectives directly as predicates.

So:

• chú thích rõ = the captions are clear

There is no need for a separate verb like is/are.

Also, Vietnamese does not use là in front of ordinary descriptive adjectives like rõ. So chú thích là rõ would be wrong here.

Rõ means clear, but in this sentence it is more about being understandable than about visual sharpness.

So it means the captions or notes become easy to understand once the guide explains them.

Depending on context, rõ can mean:

• clear
• understandable
• obvious
• easy to make out

Yes, rõ ràng is possible.

• rõ is shorter and very natural
• rõ ràng sounds a bit fuller or more explicit

So Chỉ khi hướng dẫn viên giải thích, chú thích mới rõ ràng is possible, though the original with rõ is perfectly natural.

Vietnamese often leaves out the object if it is already understood from context.

Here, the listener can infer that the guide is explaining:

• the caption itself
• the thing being referred to
• the relevant content

So the sentence does not need to say the object explicitly. This is very normal in Vietnamese.

Yes, you can say:

Chỉ khi hướng dẫn viên giải thích thì chú thích mới rõ.

That is understandable and grammatical. However, many speakers would leave out thì here because mới already does a lot of the structural work.

So:

• without thì = cleaner, very natural
• with thì = also possible, sometimes a little heavier or more spoken

The comma is not absolutely required in every situation, but it is very helpful and natural in writing.

It separates:

• the condition/time clause: Chỉ khi hướng dẫn viên giải thích
• the main clause: chú thích mới rõ

So the comma makes the sentence easier to read. In speech, this would usually be a slight pause.

Yes. It is natural and grammatical.

That said, the structure Chỉ khi ..., ... mới ... sounds a little more careful or polished than very casual speech. In everyday conversation, people might also say things like:

• Có hướng dẫn viên giải thích thì chú thích mới rõ.
• Phải nghe hướng dẫn viên giải thích thì mới hiểu chú thích.

But the original sentence is completely normal, especially in writing or more formal spoken Vietnamese.