Haec sella mihi non est idonea, sed illud scamnum commodius est.

Because the demonstratives have to agree with the nouns they modify in gender, number, and case.

• sella is feminine singular nominative, so it takes haec = this
• scamnum is neuter singular nominative, so it takes illud = that

So:

• haec sella = this chair
• illud scamnum = that bench/seat

The forms are different because the nouns belong to different genders.

Yes—both ideas are close here. Mihi is the dative singular of ego, and with idoneus/idonea/idoneum Latin often uses the dative for the person something is suitable for.

So:

• mihi non est idonea literally means is not suitable for me
• more naturally in English: isn't suitable for me or doesn't suit me

This is a very common Latin pattern: an adjective can take the dative of the person affected.

Idonea is the feminine singular nominative form of idoneus, idonea, idoneum.

It matches sella, which is:

• feminine
• singular
• nominative

Since idonea is a predicate adjective after est, it still has to agree with sella.

So:

• sella ... idonea est = the chair is suitable

If the noun were masculine, you would expect idoneus; if neuter, idoneum.

Because commodius is the neuter singular nominative comparative form, and it agrees with scamnum, which is neuter singular.

The comparative forms work like this:

• masculine/feminine nominative singular: commodior
• neuter nominative singular: commodius

So:

• illud scamnum commodius est = that bench is more comfortable/more convenient

Since scamnum is neuter, commodius is the correct form.

It can, but it doesn’t have to. Latin often leaves the second half of a comparison understood if the contrast is already obvious from the context.

Here the sentence says:

• This chair is not suitable for me, but that bench is more comfortable

The comparison is clearly between the chair and the bench, so Latin can simply use the comparative commodius without adding than this chair.

If you wanted to state it explicitly, you could say something like:

• illud scamnum commodius quam haec sella est

But that is not necessary here.

Sed means but. It introduces a contrast between the two parts of the sentence:

• Haec sella mihi non est idonea = This chair is not suitable for me
• sed illud scamnum commodius est = but that bench is more comfortable

So sed signals that the second clause contrasts with the first.

Latin word order is much more flexible than English word order because Latin uses case endings to show how words function in the sentence.

English depends heavily on position:

• The chair suits me
• Me suits the chair would sound wrong

Latin can move words around more freely because forms like mihi, haec, illud, idonea, and commodius already show their grammatical roles.

This sentence is fairly natural Latin:

• Haec sella mihi non est idonea
• sed illud scamnum commodius est

A rearranged version could still mean the same thing, though the emphasis might change.

Because non usually negates the word or idea that follows, and in a sentence like this it commonly appears before the verb:

• non est idonea = is not suitable

Latin often places non immediately before the verb, though word order can vary for emphasis.

They are related, but not identical.

• sella usually means chair, seat, or sometimes stool
• scamnum often means bench, bench-seat, or stool/seat

In this sentence, the contrast is between two different pieces of furniture, and that is part of why one is described as better for the speaker than the other.

A learner should also notice that they have different genders:

• sella = feminine
• scamnum = neuter

That affects the forms of the words that go with them.

Yes, often it could be. Latin frequently omits a repeated form of esse when it is easy to understand.

So a shortened version like:

• Haec sella mihi non est idonea, sed illud scamnum commodius

could still be understood.

However, including est makes the sentence fully explicit and clear, especially for learners.

Here is a quick breakdown:

• haec — nominative feminine singular
• sella — nominative feminine singular
• mihi — dative singular
• idonea — nominative feminine singular
• illud — nominative neuter singular
• scamnum — nominative neuter singular
• commodius — nominative neuter singular comparative

The two subjects are:

• haec sella
• illud scamnum

And mihi is in the dative because it depends on idonea in the sense suitable for me.