minna ha hakken ni odorokimasita.

みんな means everyone, everybody, or all of them.

Yes, it is somewhat casual and very common in everyday Japanese. It is not rude, but it is less formal than expressions like:

• みな = everyone
• みなさん = everyone / everyone here / ladies and gentlemen

So みんなは発見に驚きました is a normal, natural sentence, but not especially formal in tone.

は marks the topic of the sentence.

So みんなは means something like:

• as for everyone
• everyone, ...

In natural English, we usually do not translate は directly. It just tells you what the sentence is about.

So the structure is:

• みんなは = as for everyone
• 発見に = at/by the discovery
• 驚きました = was surprised / were surprised

Yes, みんなが発見に驚きました is possible, but the nuance changes.

Very roughly:

• みんなは focuses on everyone as the topic
• みんなが focuses more on everyone as the subject, often with a sense like it was everyone who was surprised

So:

• みんなは発見に驚きました = As for everyone, they were surprised by the discovery.
• みんなが発見に驚きました = Everyone was surprised by the discovery. / It was everyone who was surprised by the discovery.

For a simple neutral sentence, は is very natural.

Because the verb 驚く often uses に to mark the thing that causes surprise.

Pattern:

• N に 驚く = to be surprised at/by N

So:

• 発見に驚きました = was/were surprised at the discovery

This is just the normal grammar for 驚く.

English uses at or by, but Japanese uses に here.

No, that is not natural Japanese.

With 驚く, the thing that surprises you is usually marked with に, not を.

So this is correct:

• 発見に驚きました

If you want to use を, you usually need a different verb, such as 驚かせる:

• 発見がみんなを驚かせました
  = The discovery surprised everyone.

That sentence uses a different structure:

• 驚く = to be surprised
• 驚かせる = to surprise someone

発見 means discovery or finding.

It is a noun here.

Examples:

• 新しい発見 = a new discovery
• 大発見 = a major discovery

It can also be used as a verbal noun with する:

• 新しい星を発見した = discovered a new star

But in your sentence, 発見 is just a noun:

• the discovery
• a discovery

Which one it means depends on context, since Japanese does not use articles like a and the.

Because 驚きました is the polite past form of 驚く, which means to be surprised.

Breakdown:

• dictionary form: 驚く
• polite stem: 驚き
• polite past ending: ました
• together: 驚きました

So it means:

• was surprised
• were surprised

Since みんな means everyone/all the people, English usually translates it naturally as were surprised.

This is an important point.

In Japanese:

• 驚く = to be surprised
• 驚かせる = to surprise someone

So in this sentence:

• みんなは発見に驚きました
  = Everyone was surprised by the discovery.

But:

• 発見がみんなを驚かせました
  = The discovery surprised everyone.

English often switches between these two patterns very easily, but Japanese uses different verb forms.

Yes. The politeness comes from 驚きました.

• 驚いた = plain past, casual
• 驚きました = polite past

So this sentence is polite overall, even though みんな itself is fairly casual/common.

That kind of mix is normal in Japanese:

• everyday word + polite verb ending

Not completely. Japanese word order is more flexible than English, as long as the particles stay attached to the right words.

The most neutral order is:

• みんなは発見に驚きました

But you could also move parts around for emphasis, such as:

• 発見に、みんなは驚きました

This puts more focus on 発見に.

Still, the verb usually comes at the end, and the standard learner-friendly order is the one you were given.

Because Japanese does not have articles like English a, an, and the.

So 発見 could mean:

• a discovery
• the discovery

The context tells you which is meant.

That is why one Japanese sentence can match several natural English translations.

No. Japanese is normally written without spaces.

So the normal writing would be:

• みんなは発見に驚きました。

Spaces are often added in beginner materials to make the parts easier to see.

It is read:

みんな は はっけん に おどろきました。

More naturally without spacing:

みんなは はっけんに おどろきました。

Word readings:

• みんな = minna
• 発見 = hakken
• 驚きました = odorokimashita

So the full pronunciation is roughly:

minna wa hakken ni odorokimashita