Ĉi tiu sako estas reuzebla.

In Esperanto, tiu means that one / that, and ĉi is a little particle that brings the meaning closer, like here or this.

So:

• tiu = that
• ĉi tiu = this

You can think of ĉi as turning a pointing word into something near the speaker.

Examples:

• tiu sako = that bag
• ĉi tiu sako = this bag

You may also see tiu ĉi, which means the same thing.

Yes. Both are correct:

• ĉi tiu sako
• tiu ĉi sako

Both mean this bag.

Ĉi tiu is probably the more common order for many learners, but both are normal Esperanto.

Because -o is the noun ending in Esperanto.

So:

• sako = bag
• libro = book
• domo = house

Esperanto uses regular endings to show parts of speech:

• -o = noun
• -a = adjective
• -e = adverb
• -i = infinitive verb

That makes it easier to recognize the job of each word.

Because Esperanto verbs do not change according to the subject.

In English, we say:

• I am
• you are
• it is

But in Esperanto, the present tense of to be is always estas:

• mi estas = I am
• vi estas = you are
• ĝi estas = it is
• ĉi tiu sako estas = this bag is

So estas stays the same no matter who or what the subject is.

Reuzebla is made from several parts:

• re- = again
• uz = use
• -ebl- = able to be, possible to
• -a = adjective

So:

re- + uz + ebl + a = reuzebla

This gives the idea able to be used again, or reusable.

This is a very Esperanto-like way of building vocabulary: one root plus clear affixes.

-ebla means able to be or can be.

Examples:

• legebla = readable = able to be read
• manĝebla = edible = able to be eaten
• videbla = visible = able to be seen
• reuzebla = reusable = able to be used again

So reuzebla does not just mean used again. It means the bag can be used again.

Because it is an adjective, and adjectives in Esperanto end in -a.

Here, reuzebla describes sako, so it has to be an adjective:

• sako = bag
• reuzebla sako = reusable bag

In the sentence Ĉi tiu sako estas reuzebla, the word reuzebla is a predicate adjective: it describes what the bag is like.

Because there is no direct object here.

The sentence structure is:

• Ĉi tiu sako = subject
• estas = linking verb
• reuzebla = adjective describing the subject

The accusative -n is usually used for a direct object, and there is no direct object in this sentence.

So:

• Ĉi tiu sako estas reuzebla. = no -n

But compare:

• Mi uzas ĉi tiun sakon. = I use this bag.

There, ĉi tiun sakon gets -n because it is the direct object.

Actually, it already matches sako.

Since sako is singular and not accusative, the matching forms are:

• ĉi tiu sako

If the noun changed, the demonstrative would usually change too:

• plural: ĉi tiuj sakoj = these bags
• accusative singular: ĉi tiun sakon = this bag
• accusative plural: ĉi tiujn sakojn = these bags

So Esperanto often makes the pointing word agree with the noun in number and case.

A simple pronunciation guide is:

• Ĉi ≈ chee
• tiu ≈ TEE-oo
• sako ≈ SAH-ko
• estas ≈ ES-tahs
• reuzebla ≈ reh-oo-ZEB-lah

A few useful pronunciation notes:

• ĉ sounds like ch in church
• u sounds like oo in food
• stress in Esperanto normally falls on the second-to-last syllable

So:

• SA-ko
• ES-tas
• re-u-ZE-bla

That would not be the most natural way to say it.

Ĉi tiu sako estas reuzebla means This bag is reusable, which is a normal full sentence.

If you say reuzebla sako, that is a noun phrase meaning a reusable bag. For example:

• Ĉi tio estas reuzebla sako. = This is a reusable bag.

So:

• Ĉi tiu sako estas reuzebla. = This bag is reusable.
• Ĉi tio estas reuzebla sako. = This is a reusable bag.

Notice the difference between ĉi tiu (this one / this [noun]) and ĉi tio (this [thing]).

This is a very common question.

• ĉi tiu = this one / this and usually points to a specific noun
• ĉi tio = this in a more general sense, like this thing

Examples:

• Ĉi tiu sako estas reuzebla. = This bag is reusable.
• Ĉi tio estas sako. = This is a bag.

So in your sentence, ĉi tiu is used because it directly goes with sako.

Normally, no.

Esperanto usually requires the verb estas in this kind of sentence, just as English needs is:

• Ĉi tiu sako estas reuzebla. = correct

Without estas, you would just have a noun phrase plus an adjective, not a complete normal sentence:

• Ĉi tiu sako reuzebla = incomplete or unnatural in standard usage

So for a basic statement, keep estas.