Learn Hardcore Esperanto: Tiu solvo estas bona por ni.

In Esperanto, tiu is a demonstrative adjective/pronoun used with a noun, like tiu solvo = that solution.
Tio is a stand‑alone pronoun and cannot directly modify a noun, so tio solvo is wrong; you would say tio estas bona solvo (that is a good solution).

Ĉi tiu solvo means this solution (here) and adds a feeling of physical or conceptual closeness. In many contexts you don’t need to emphasize “this” vs “that”, so simple tiu solvo is enough and is very common.

Why is there no la (the) before solvo?

The word tiu already makes the noun specific: tiu solvo literally means that solution (a particular one). Because tiu is a determiner, you normally don’t add la as well; la tiu solvo is ungrammatical in standard Esperanto.

If you want the solution without pointing to it as “this/that one”, you could say:

La solvo estas bona por ni. – The solution is good for us.

What exactly does por mean here, and how is it different from al?

Por basically means for, especially in the sense of:

benefit or advantage: bona por ni – good for us
purpose: por vi – for you, por labori – in order to work

Al means to (direction, recipient):

Donu ĝin al ni. – Give it to us.
Li parolas al mi. – He speaks to me.

So bona por ni = good for us / beneficial to us, not bona al ni.

Why is it bona and not bonan?

The ending -n marks the accusative, usually the direct object of a verb or a direction of movement. After estas (to be), there is no direct object; bona is just a predicate adjective describing solvo. Predicate adjectives and predicate nouns do not take -n:

La solvo estas bona. – The solution is good.
Li estas instruisto. – He is a teacher. (no instruiston)

You would use -n with bona only when the whole noun phrase needs the accusative, e.g.:

Mi trovis bonan solvon. – I found a good solution.

Why does bona end in -a and not -e or something else?

In Esperanto:

-a is the ending for adjectives: bona, granda, rapida.
-e is the ending for adverbs: bone, grande, rapide.

So bona means good (adjective), describing a noun: bona solvo.
Bone means well (adverb), describing a verb: Li solvas bone. – He solves (it) well.

In Tiu solvo estas bona por ni, bona describes the noun solvo, so it must use -a.

Why does bona go after estas instead of before solvo, like in English “that good solution is for us”?

The sentence is using a “X is Y” structure:

Tiu solvo – that solution (subject)
estas bona – is good (predicate)

So the pattern is [subject] + estas + [description of the subject]. That’s the most natural way to say “That solution is good”.

If you want “that good solution” as a single noun phrase, you would say:

Tiu bona solvo estas por ni. – That good solution is for us.

Both are correct, but they mean slightly different things, just like in English.

Why is it ni and not nin after por?

In Esperanto, prepositions take the nominative form of pronouns, without -n:

por mi, por vi, por li, por ŝi, por ni, por ili

The accusative -n is not triggered just by a preposition; it’s mostly for direct objects and some directional uses. So por ni is correct.
Por nin would be wrong here.

What tense is estas, and does it change with different subjects?

Estas is the present tense of esti (to be). Esperanto verbs do not change with the subject; the present form is always -as:

Mi estas – I am
Vi estas – you are
Li/ŝi/ĝi estas – he/she/it is
Ni estas – we are
Ili estas – they are

The past would be estis, and the future estos, regardless of the subject.

How would the sentence change if we talk about several solutions?

To make it plural, you add -j to nouns and their agreeing adjectives/demonstratives:

Tiuj solvoj estas bonaj por ni.
- tiu → tiuj (that → those)
- solvo → solvoj (solution → solutions)
- bona → bonaj (good → good, plural)

Everything that “belongs” to the noun (demonstratives and adjectives) must agree in number with it.

Could I say the same idea in another common way?

Yes, a few natural alternatives are:

Tiu solvo taŭgas por ni. – That solution suits us / is suitable for us.
Tiu solvo plaĉas al ni. – We like that solution.
Tiu solvo estas konvena por ni. – That solution is convenient/appropriate for us.

Each has a slightly different nuance, but all keep the same basic idea.

Can I move por ni to another place in the sentence?

Yes. Esperanto word order is quite flexible because roles are mostly shown by endings, not position. All of these are grammatical, with slight differences in emphasis:

Tiu solvo estas bona por ni. (neutral)
Por ni tiu solvo estas bona. (emphasizes for us)
Tiu solvo, por ni, estas bona. (spoken, with a pause for emphasis)

The most standard, neutral style is the original: Tiu solvo estas bona por ni.