Khi thấy món quà, Lan đã cười rất nhiều.

Khi means when and introduces a time clause.

So Khi thấy món quà means something like When (she) saw the gift.

A very common pattern is:

Khi + clause, main clause

For example:

• Khi về nhà, tôi ăn cơm. = When I got home, I ate.
• Khi nghe tin đó, cô ấy khóc. = When she heard that news, she cried.

In your sentence, the khi clause sets the time/background for the main action.

Vietnamese often leaves out the subject when it is already understood from context.

In Khi thấy món quà, Lan đã cười rất nhiều, the person who saw the gift is understood to be Lan. So Vietnamese does not need to repeat her name.

A more explicit version would be:

Khi Lan thấy món quà, Lan đã cười rất nhiều.

But that sounds repetitive. The original is much more natural.

This is very common in Vietnamese, especially when both clauses have the same subject.

Thấy usually means to see, to notice, or to perceive.

Here it means saw or caught sight of the gift.

It is a very common verb and often works differently from English in some contexts. For example:

• Tôi thấy anh ấy. = I saw him.
• Tôi thấy mệt. = I feel tired / I find myself tired.
• Tôi thấy việc này khó. = I find this difficult.

In your sentence, it has the straightforward visual meaning: to see.

Món is a classifier-like word often used with certain objects, including gifts, dishes, and some individual items.

So món quà is a very common way to say a gift / present.

You may also hear just quà in some contexts, but món quà sounds very natural and complete when referring to a specific gift.

Compare:

• quà = gift(s), presents in a general sense
• món quà = a gift, a present, one gift item

So here, món quà is not strange or overly formal; it is a standard expression.

Đã marks the action as completed or as happening in the past context.

In Lan đã cười rất nhiều, it tells you that Lan’s laughing happened as a completed event after seeing the gift.

Important point: Vietnamese does not have tense in the same way English does. Words like đã, đang, and sẽ help show time or aspect:

• đã = past/completed
• đang = in progress
• sẽ = future

So:

• Lan cười rất nhiều = Lan laughs/laughed a lot, depending on context
• Lan đã cười rất nhiều = Lan laughed a lot / Lan did laugh a lot

In many cases, đã can be omitted if the time is already clear, but including it makes the past/completed meaning clearer.

With verbs, rất nhiều often means a lot or very much.

So:

cười rất nhiều = laughed a lot

Even though rất often means very, the whole phrase rất nhiều works naturally as an intensifier meaning a great deal.

Compare:

• đọc rất nhiều = read a lot
• học rất nhiều = study a lot
• cười rất nhiều = laugh a lot

You could also say just cười nhiều, which also means laugh a lot, but rất nhiều makes it stronger.

Yes, it can be reversed.

The original:

Khi thấy món quà, Lan đã cười rất nhiều.

A reversed version:

Lan đã cười rất nhiều khi thấy món quà.

Both are natural. The difference is mostly one of focus and style:

• Khi thấy món quà, ... puts the time/background first.
• Lan đã cười rất nhiều khi thấy món quà. starts with the main subject and action first.

Vietnamese is flexible with this kind of time clause, just like English.

It can be omitted.

You can say:

Khi thấy món quà, Lan cười rất nhiều.

This is still understandable, and the past meaning can come from context. But đã makes the sentence more explicitly about a completed past event.

So:

• with đã = clearer past/completed action
• without đã = still possible, but more dependent on context

This is normal in Vietnamese. Time is often understood from context rather than always marked directly.

No. It means she laughed a lot, not necessarily loudly.

That is an important distinction.

• cười rất nhiều = laughed a lot
• cười to = laughed loudly
• cười lớn = laughed loudly / gave a loud laugh

So in your sentence, the focus is on the amount or extent of laughing, not the volume.

Yes, in many contexts you could say:

Khi nhìn thấy món quà, Lan đã cười rất nhiều.

This also means When she saw the gift, Lan laughed a lot.

The difference is subtle:

• thấy = see, notice
• nhìn thấy = literally look and see, often a bit more explicit about the act of seeing

In everyday Vietnamese, thấy is very common and perfectly natural here. The original sentence sounds smooth and idiomatic.

With đã, it strongly suggests a specific completed event in the past.

So the sentence is most naturally understood as:

At that moment, when Lan saw the gift, she laughed a lot.

If you wanted a habitual meaning, you would usually rely on different context or wording. As written, this sounds like one past event, not a repeated pattern.

The sentence can be broken down like this:

Khi thấy món quà,
= when seeing the gift / when she saw the gift

Lan
= Lan

đã cười rất nhiều
= laughed a lot

So the structure is:

Time/background clause + subject + main verb phrase

More abstractly:

Khi + verb phrase, Subject + đã + verb + degree phrase

That pattern is very common in Vietnamese and useful to learn.