Tiu folio estas verda.

Tiu means that.

It is a demonstrative word, used to point something out. In this sentence, tiu folio means that leaf.

A useful contrast:

• tiu = that
• ĉi tiu = this

So:

• Tiu folio estas verda. = That leaf is green.
• Ĉi tiu folio estas verda. = This leaf is green.

In Esperanto, nouns normally end in -o.

So:

• folio = leaf
• domo = house
• hundo = dog

That -o ending tells you right away that folio is a noun.

In Esperanto, adjectives normally end in -a.

So:

• verda = green
• bela = beautiful
• granda = big

Since verda is describing folio, it takes the adjective ending -a.

Because green here is an adjective, not an adverb.

In Esperanto:

• -a = adjective
• -e = adverb

So:

• verda = green
• verde = greenly / in a green way

After estas, Esperanto usually uses an adjective when describing what something is like:

• La ĉielo estas blua. = The sky is blue.
• La folio estas verda. = The leaf is green.

So verda is correct.

Estas means is / am / are.

It is the present-tense form of esti = to be.

One very convenient feature of Esperanto is that verbs do not change depending on the subject:

• mi estas = I am
• vi estas = you are
• li estas = he is
• ŝi estas = she is
• ĝi estas = it is
• ili estas = they are

So in Tiu folio estas verda, estas simply means is.

Because tiu already makes the noun specific.

In Esperanto, you do not say la tiu folio. That would be wrong.
You just say:

• tiu folio = that leaf

Similarly:

• ĉi tiu libro = this book
• tiu domo = that house

The demonstrative already does the job of identifying the noun.

Yes. In Esperanto, adjectives agree with the nouns they describe.

In this sentence, both are singular and neither has -n, so:

• folio
• verda

If the noun changed, the adjective would change too:

• Tiu folio estas verda. = That leaf is green.
• Tiuj folioj estas verdaj. = Those leaves are green.

And if the noun were accusative, the adjective would also take -n:

• Mi vidas tiun verdan folion. = I see that green leaf.

So adjective agreement is important in Esperanto.

Because this sentence has no direct object.

The noun folio is the subject of the sentence, not the object. It is the thing being described.

In Esperanto, -n is usually used for the direct object:

• Mi vidas la folion. = I see the leaf.

But in:

• Tiu folio estas verda.

the word folio is not being acted on. It is just the subject, so there is no -n.

Also, after estas, the describing adjective stays as an adjective:

• verda, not verdan

Yes, Esperanto word order is fairly flexible, although Tiu folio estas verda is the most neutral and natural order.

You may also see:

• Verda estas tiu folio.

This puts more emphasis on verda.

Still, for learners, the clearest basic pattern is:

• subject + verb + adjective
• Tiu folio estas verda.

A rough guide for an English speaker is:

• Tiu ≈ TEE-oo
• folio ≈ fo-LEE-o
• estas ≈ ES-tahs
• verda ≈ VER-dah

A few helpful pronunciation notes:

• tiu is two syllables: ti-u
• Esperanto vowels are pronounced clearly and separately
• the stress is usually on the second-to-last syllable:
  • foLIo
  • EStas
  • VERda

Yes. If the context is clear, tiu can stand on its own and mean that one.

For example:

• Tiu estas verda. = That one is green.

But in your sentence, folio is included to say exactly what thing is being talked about:

• Tiu folio estas verda. = That leaf is green.

You would make both the noun and the adjective plural, and usually the demonstrative too:

• Tiuj folioj estas verdaj. = Those leaves are green.

Changes:

• tiu → tiuj
• folio → folioj
• verda → verdaj

This shows again that adjectives agree with the noun they describe.