A meglepetésbuli nagy meglepetés lesz.

Because it is a Hungarian compound noun: meglepetés + buli.

Hungarian very often writes compounds as a single word, especially when English would use something like surprise party, school bus, coffee cup, and so on.

In this compound:

• meglepetés = surprise
• buli = party

So meglepetésbuli literally means surprise-party.

A useful pattern to notice is that the last part often tells you what kind of thing it is. Here it is a kind of buli.

A is the definite article, so it corresponds to English the.

Hungarian has two forms of the definite article:

• a before a consonant sound
• az before a vowel sound

Since meglepetésbuli starts with m, a consonant, you use a:

• a meglepetésbuli

If the next word started with a vowel, you would use az instead.

This is very common in Hungarian.

After van / lesz when a noun phrase is used as a description or classification, Hungarian often leaves out the indefinite article egy, even where English would use a.

So:

• nagy meglepetés lesz = will be a big surprise

You could also say:

• A meglepetésbuli egy nagy meglepetés lesz.

That is also grammatical, but egy adds a bit more emphasis, a bit like will be one big surprise or will be quite a surprise depending on context.

So the version without egy is very normal.

Here lesz means will be.

It is the future form of van (to be). So:

• van = is / exists
• lesz = will be

In this sentence:

• A meglepetésbuli nagy meglepetés lesz.
• The surprise party will be a big surprise.

In other contexts, lesz can also have a sense closer to become, but here will be is the best way to understand it.

Usually, no.

In Hungarian, in the 3rd person singular present tense with a noun or adjective predicate, van is normally omitted.

So the present-tense version would usually be:

• A meglepetésbuli nagy meglepetés.

Not:

• A meglepetésbuli nagy meglepetés van. ❌

But in the future, you do need lesz:

• A meglepetésbuli nagy meglepetés lesz.

This is a very important Hungarian pattern.

Nagy is an adjective meaning big / great, and in Hungarian adjectives normally come before the noun they describe.

So:

• nagy meglepetés = big surprise

Also, Hungarian adjectives usually do not agree with the noun the way they do in many Indo-European languages. That means the adjective often stays in its basic form.

So even though English learners may expect some extra ending, here plain nagy is exactly what you want.

Yes, it is normal enough, and here it is probably a bit playful.

You have:

• meglepetésbuli = surprise party
• meglepetés = surprise

So the sentence has a kind of deliberate repetition:

• The surprise party will be a big surprise.

That sounds a little rhetorical or humorous, but it is perfectly understandable. Hungarian, like English, can repeat a word for effect.

Yes. Hungarian word order is flexible, and changing it often changes the focus or emphasis.

The original:

• A meglepetésbuli nagy meglepetés lesz.

This is a fairly neutral structure: the topic is the surprise party, and then you say something about it.

You could also say:

• Nagy meglepetés lesz a meglepetésbuli.

This puts more focus on nagy meglepetés — basically It’s going to be a big surprise, the surprise party.

So the original is natural, but not the only possible order.

Buli is somewhat informal or conversational. It is a very common everyday word for party.

So meglepetésbuli sounds natural in normal speech.

Depending on context, Hungarian can also use other words like parti or more formal event words, but buli is very common and idiomatic here.

So for a sentence about a surprise party, meglepetésbuli sounds perfectly normal.

A rough pronunciation guide is:

aw MEG-leh-peh-taysh-boo-lee nawdj MEG-leh-peh-taysh less

A few useful points:

• Hungarian stress is normally on the first syllable of each word.
• sz is pronounced like English s.
• s is pronounced like English sh.
• é is a long vowel, roughly like the vowel in say, but without the English glide.
• lesz sounds close to English less.

One slightly advanced point: in natural speech, the s sound at the end of meglepetés may sound more voiced before b in buli, so learners may hear something a bit like meglepetézhbuli. But you do not need to worry much about that at first; the spelling stays meglepetésbuli.