Hôm nay là sinh nhật của Lan, nên tôi muốn tặng cô ấy một món quà đẹp.

Here là works like is in English.

• Hôm nay = today
• là = is
• sinh nhật của Lan = Lan’s birthday

So the structure is basically:

Today is Lan’s birthday.

In this kind of identifying sentence, là is very natural and standard.

Của is a common way to show possession, similar to of or the English ’s.

So:

• sinh nhật của Lan = Lan’s birthday
• literally: birthday of Lan

A very common pattern is:

noun + của + owner

Examples:

• nhà của tôi = my house
• bạn của Lan = Lan’s friend

In casual Vietnamese, people sometimes leave out của when the meaning is still clear, but using của is safe and clear for learners.

Nên means so, therefore, or that’s why.

It connects the first idea to the result:

• Hôm nay là sinh nhật của Lan = Today is Lan’s birthday
• nên tôi muốn tặng cô ấy một món quà đẹp = so I want to give her a beautiful gift

So the logic is:

Because it’s Lan’s birthday, I want to give her a beautiful gift.

You will often see nên used for results or consequences.

Tôi is a neutral, polite, and common way to say I.

It is often used:

• in standard written Vietnamese
• when speaking politely
• when the relationship is neutral or not very close

In real conversation, Vietnamese often uses different pronouns depending on age, relationship, and social role. So instead of tôi, a speaker might say mình, em, anh, chị, etc., depending on who they are talking to.

But tôi is a very good standard form for learners.

Cô ấy means she or her.

• cô = a female person, often translated as she in this kind of sentence
• ấy = that, that one

Together, cô ấy means she/her.

In this sentence, it refers to Lan.

Vietnamese often repeats the person’s name instead of using a pronoun, so you could also say:

... tôi muốn tặng Lan một món quà đẹp.

That is also natural.

A useful note: Vietnamese pronouns are more socially specific than English pronouns, so the best choice can depend on age and relationship. But cô ấy is a normal written way to say she/her.

Tặng means to give as a gift or to present.

That makes it a better match than plain cho, which just means give.

Compare:

• tặng = give as a present
• cho = give

So because the sentence is about a birthday present, tặng is the natural choice.

Also, Vietnamese often uses this pattern:

tặng + person + thing

So:

tặng cô ấy một món quà đẹp
= give her a beautiful gift

You may also hear:

tặng cho cô ấy một món quà đẹp

The extra cho is possible, but it is often omitted because tặng already implies giving to someone.

Vietnamese often allows this pattern:

verb + recipient + thing

So:

• tặng cô ấy một món quà đẹp
• literally: give her a beautiful gift

This is similar to the English pattern give someone something.

Another possible order is:

tặng một món quà đẹp cho cô ấy

This is more like English give a beautiful gift to her.

Both are correct. The sentence you were given uses the very common pattern with the recipient first.

Một literally means one, but it often works like a/an in English.

So:

• một món quà đẹp = a beautiful gift

It does not always strongly emphasize the number one. Very often it just introduces a singular countable noun.

Món is a classifier.

Vietnamese often uses classifiers when a noun is counted or introduced with words like một.

So:

• món quà = a gift / present
• một món quà = one gift / a gift

In this case, món is the natural classifier for something like a gift, item, or offering.

Also, món quà is a very common set phrase in Vietnamese. It sounds much more natural than trying to use quà by itself after một.

In Vietnamese, adjectives usually come after the noun they describe.

So:

• món quà đẹp
• literally: gift beautiful
• natural English: beautiful gift

This is one of the most important word-order differences from English.

More examples:

• nhà đẹp = beautiful house
• người tốt = good person
• xe mới = new car

Not by changing the verb form.

Vietnamese verbs do not conjugate the way English verbs do. Instead, time is usually understood from context, time words, or extra markers.

In this sentence:

• Hôm nay tells you the time: today
• muốn tells you it is a present desire or intention: want

So tôi muốn tặng... means I want to give...

The action of giving may happen very soon or later that day, and the context makes that understandable without a special future verb ending.

Yes. The original sentence is correct and clear, but Vietnamese often allows simpler versions.

For example:

Hôm nay là sinh nhật của Lan, nên tôi muốn tặng Lan một món quà đẹp.
This repeats Lan instead of using cô ấy.

You might also hear:

Hôm nay là sinh nhật Lan, nên tôi muốn tặng cô ấy một món quà đẹp.
This is more casual because của is omitted.

Or even:

Hôm nay sinh nhật Lan, tôi muốn tặng cô ấy một món quà đẹp.
This is even more conversational.

For learners, though, the original sentence is a very good standard model.

The comma helps separate the reason from the result:

• reason: Hôm nay là sinh nhật của Lan
• result: nên tôi muốn tặng cô ấy một món quà đẹp

In writing, the comma makes the sentence easier to read. In speech, there is often a small pause there.

It is not about a special grammar rule as strict as in some English writing, but it is very normal and helpful punctuation.