Tôi ghi số hiệu chuyến bay.

Ghi usually means to note down, record, write down, or jot down. In this sentence, it suggests writing something down for reference.

It is related to viết (to write), but they are not always the same:

• viết = the general act of writing
• ghi = writing something down in order to record or remember it

So Tôi ghi số hiệu chuyến bay sounds more like I note down the flight number than simply I write the flight number.

Because Vietnamese often expresses that idea with a single verb. Ghi already includes the sense of writing something down / recording it.

So English needs two words:

• write down

But Vietnamese can often use just one:

• ghi

It can be broken down like this:

• số = number
• hiệu = designation, code, identifying mark
• chuyến bay = flight

Together, số hiệu chuyến bay means flight number or flight code/identifier.

This is a fairly formal, precise way to say it.

Vietnamese noun phrases often work from the general identifying noun to the thing being identified.

Here:

• số hiệu = identifying number/code
• chuyến bay = flight

So số hiệu chuyến bay literally feels like the identifying number of the flight.

This kind of noun stacking is very common in Vietnamese.

Sometimes people may understand số chuyến bay, but số hiệu chuyến bay is more precise and standard for flight number.

Another very common modern alternative is:

• mã chuyến bay = flight code

So depending on context, you may hear:

• số hiệu chuyến bay
• mã chuyến bay

Both can refer to the flight number/code, though mã chuyến bay is very common in everyday usage.

Vietnamese does not use articles the same way English does. There is usually no direct equivalent of a/an or the in ordinary sentences like this.

So:

• Tôi ghi số hiệu chuyến bay

can mean:

• I write down a flight number
• I write down the flight number

Which one is meant depends on context.

That is normal in Vietnamese. Verbs do not change form for tense the way English verbs do.

So ghi stays ghi whether the action is present, past, or future. Time is usually understood from:

• context
• time words, such as hôm qua (yesterday), bây giờ (now), ngày mai (tomorrow)
• aspect markers, such as đã, đang, sẽ

Examples:

• Tôi ghi số hiệu chuyến bay. = I write down / am writing down / wrote down the flight number.
• Tôi đang ghi số hiệu chuyến bay. = I am writing down the flight number.
• Tôi đã ghi số hiệu chuyến bay. = I wrote / have written down the flight number.
• Tôi sẽ ghi số hiệu chuyến bay. = I will write down the flight number.

This follows the basic Vietnamese word order:

• Subject + Verb + Object

So:

• Tôi = I
• ghi = write down / record
• số hiệu chuyến bay = the flight number

That makes the sentence structure very similar to English:

• I + write down + the flight number

No. Tôi is a neutral, common way to say I, especially in standard Vietnamese and when the relationship between speakers is not especially close or socially marked.

Depending on who you are speaking to, you might use other pronouns, such as:

• mình
• em
• anh
• chị
• con
• cháu
• tớ

Vietnamese pronouns depend heavily on age, relationship, politeness, and social role. But tôi is a safe default in many situations.

Yes, it is grammatical and natural, especially in a neutral or somewhat formal style.

In casual conversation, people might also say things like:

• Tôi ghi lại số chuyến bay.
• Tôi ghi lại mã chuyến bay.
• Tôi note lại mã chuyến bay. (more colloquial, influenced by English in some contexts)

But Tôi ghi số hiệu chuyến bay is perfectly correct and clear.

Ghi lại often adds the sense of writing something down so you can keep it, remember it, or refer to it later.

So:

• ghi = note down / record
• ghi lại = write down / note down and keep a record of it

In many contexts, the difference is small.
For this sentence, both could work:

• Tôi ghi số hiệu chuyến bay.
• Tôi ghi lại số hiệu chuyến bay.

The version with lại can sound a little more explicitly like I write it down for later reference.

A rough pronunciation guide is:

• Tôi ≈ toy
• ghi ≈ ghee (with a hard g sound in the South; in the North it may sound closer to z/gh depending on accent, but learners are usually taught a standard ghi pronunciation)
• số ≈ soh with a rising broken tone
• hiệu ≈ something like hee-uh with a falling/glottal tone on the first part depending on accent
• chuyến ≈ roughly chwien
• bay ≈ bye

A more useful thing than English approximation is to pay attention to the tones, because they matter a lot:

• tôi – ngang
• ghi – ngang
• số – sắc
• hiệu – nặng on hiệu
• chuyến – sắc
• bay – ngang

If you want to sound natural, the tones are more important than trying to match English sounds exactly.