Meidän täytyy suojella luontoa.

Because in this structure, Finnish does not use the normal nominative subject me.

With täytyy (must / have to), the person who has the obligation is usually put in the genitive case:

• minun täytyy = I must
• sinun täytyy = you must
• meidän täytyy = we must
• heidän täytyy = they must

So meidän here does not mean our. It is the genitive form of me, used in this necessity structure.

Täytyy comes from the verb täytyä, but in modern Finnish it behaves in a fairly special way.

In sentences like this, it is used almost only in the 3rd person singular form:

• minun täytyy
• sinun täytyy
• meidän täytyy

So you do not normally change it to match the person the way you would with many other verbs. You do not say things like me täydymme for this meaning.

A good way to think of the pattern is:

[genitive person] + täytyy + infinitive

So:

• Meidän täytyy suojella luontoa.

Because after täytyy, the main action is given in the basic infinitive form.

So:

• täytyy mennä = must go
• täytyy tehdä = must do
• täytyy suojella = must protect

Here, suojella means to protect, and it stays in that infinitive form after täytyy.

Because luontoa is the partitive form of luonto.

In Finnish, objects often appear in either the partitive or the total object form, depending on meaning. Here the partitive is natural because protecting nature is understood as an ongoing, general activity, not as a single completed action with a clear endpoint.

So:

• suojella luontoa = protect nature

The partitive often appears when the object is:

• general,
• unbounded,
• ongoing,
• or not seen as fully completed.

That fits this sentence well: nature here is a broad, general idea.

Normally, no.

Luonnon is not the form you would usually use here. In this sentence, Finnish strongly prefers luontoa.

That is because suojella commonly takes a partitive object when talking about protecting something in a general sense:

• suojella luontoa
• suojella ympäristöä
• suojella eläimiä

Using a non-partitive object here would sound unnatural or would at least change the nuance a lot.

Not in the same way as in English.

In English, we is the grammatical subject of must protect.

In Finnish, with täytyy, the person who experiences the necessity is usually in the genitive:

• meidän = for us / we (in this structure)

So meidän is the logical we, but grammatically this construction is different from a normal subject-verb sentence.

That is one reason this pattern can feel unusual to English speakers.

No. Even though meidän can mean our in other contexts, that is not what it means here.

Compare:

• meidän talo = our house
• meidän täytyy lähteä = we must leave

So in this sentence, meidän is part of the grammar of obligation, not a possessive adjective.

Yes. A very common alternative is:

• Meidän pitää suojella luontoa.

This also means We must / have to protect nature.

For many learners, pitää and täytyy are very similar in meaning in this kind of sentence. In everyday language, pitää is extremely common.

Both use the same kind of pattern:

• meidän pitää tehdä se
• meidän täytyy tehdä se

The difference is usually not huge in basic usage.

Yes, if you want a more general meaning.

• Täytyy suojella luontoa.

This means something like:

• Nature must be protected
• One must protect nature
• We/people must protect nature

Without meidän, the sentence becomes less specifically about us and more general or impersonal.

Finnish word order is fairly flexible, but the neutral order here is:

• Meidän täytyy suojella luontoa.

You might also hear other orders for emphasis, for example:

• Luontoa meidän täytyy suojella.

That puts more focus on nature.

Still, for a learner, the basic order is the safest choice: person in genitive + täytyy + infinitive + object

A rough pronunciation guide is:

• täy-tyy

A few points:

• ä is like a more open front vowel, somewhat like the vowel in British cat, but not exactly the same.
• y is a rounded front vowel; English does not really have it.
• yy is a long vowel, so hold it longer.

Also, Finnish stress usually falls on the first syllable:

• TÄY-tyy

If pronunciation is hard, that is very normal: y is one of the trickier Finnish vowels for English speakers.

Usually you would not negate täytyy directly in the most natural everyday Finnish. Instead, Finnish commonly switches to another expression:

• Meidän ei tarvitse suojella luontoa. = We do not need to protect nature.
• Meidän ei ole pakko suojella luontoa. = We do not have to protect nature.

Be careful: these mean lack of necessity, not we must not.

If you want we must not protect nature, that is a different meaning and would need a different expression.

It is basically neutral standard Finnish.

It works well in:

• writing,
• school contexts,
• news-style language,
• careful speech.

In everyday conversation, many people might also say:

• Meidän pitää suojella luontoa.

But Meidän täytyy suojella luontoa is completely normal and correct.