Haec sella commodior est quam illud scamnum, quia avia diu ibi sedere potest.

Because haec has to agree with sella in gender, number, and case.

• sella = a feminine singular nominative noun
• so the demonstrative must also be feminine singular nominative
• that form is haec

The three nominative singular forms are:

• hic = this (masculine)
• haec = this (feminine)
• hoc = this (neuter)

So haec sella means this chair.

For the same reason: illud agrees with scamnum.

• scamnum is neuter singular nominative
• so the demonstrative must be neuter singular nominative
• that form is illud

So:

• illa sella = that chair
• illud scamnum = that bench / stool / seat

This is a very common Latin pattern: demonstratives match the noun they describe.

They are two different nouns for kinds of seats.

• sella usually means chair
• scamnum usually means bench, stool, or a more basic kind of seat

In this sentence, the comparison is between two different pieces of furniture:

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

A learner may notice that English often uses seat loosely, but Latin is being more specific here.

Commodior is the comparative of commodus: more comfortable, more convenient, or more suitable, depending on context.

Comparative adjectives in Latin work a little differently from ordinary first/second-declension adjectives. Their nominative singular forms are:

• commodior = masculine
• commodior = feminine
• commodius = neuter

Since sella is feminine singular, the correct form is commodior.

So:

• sella commodior = a more comfortable chair
• scamnum commodius = a more comfortable bench/stool

It comes from the positive adjective commodus, -a, -um.

The comparison goes like this:

• commodus = comfortable / suitable
• commodior = more comfortable / more suitable
• commodissimus = most comfortable / most suitable

So commodior is simply the comparative degree.

This is a very common pattern:

• altus → altior = higher
• gravis → gravior = heavier / more serious
• commodus → commodior = more comfortable

Quam is the normal word for than in a comparison.

So:

• commodior est quam illud scamnum
• literally: is more comfortable than that bench

This is the standard Latin pattern:

• X maior est quam Y = X is bigger than Y
• X melior est quam Y = X is better than Y
• X commodior est quam Y = X is more comfortable than Y

In this construction, the thing after quam is often in the same case as the thing being compared. Here both are effectively nominative ideas:

• haec sella
• illud scamnum

Here est is the normal and expected form.

• commodior est = is more comfortable

Latin sometimes omits forms of sum in certain styles, especially poetry or very compressed expression, but in ordinary prose you normally keep it. So this sentence is perfectly straightforward and natural with est.

Avia is nominative singular.

It is the subject of potest:

• avia ... potest = grandmother can ...

So in the clause

• quia avia diu ibi sedere potest

the subject is avia, and the verb is potest.

That whole clause explains why the chair is more comfortable.

Latin commonly expresses can with possum plus an infinitive.

So:

• potest = is able
• sedere = to sit
• sedere potest = can sit

Literally, it is something like grandmother is able to sit there for a long time.

This is exactly parallel to many other Latin expressions:

• venire potest = he/she can come
• videre possum = I can see
• facere possumus = we can do/make

Because it depends on potest.

After possum, Latin normally uses the present active infinitive of the main action:

• potest sedere = can sit
• potest currere = can run
• potest dormire = can sleep

So sedere is not the main finite verb of the clause. The finite verb is potest.

Diu means for a long time or a long time.

It is an adverb, so it modifies the action:

• sedere potest = can sit
• diu sedere potest = can sit for a long time

So the idea is that the chair is more comfortable because grandmother can remain seated there for a long time.

Ibi means there.

It is an adverb of place, not an adjective. It tells you where the sitting happens:

• ibi sedere potest = she can sit there

In context, there refers to the relevant seat being discussed. Depending on how the sentence is being understood in context, it most naturally points to the chair that has just been called more comfortable.

Quia means because.

It introduces the reason:

• Haec sella commodior est quam illud scamnum = This chair is more comfortable than that bench.
• quia avia diu ibi sedere potest = because grandmother can sit there for a long time.

So the second clause explains the basis of the comparison.

Latin word order is fairly flexible because the endings already show the grammatical relationships.

This sentence is perfectly natural as written:

• Haec sella commodior est quam illud scamnum, quia avia diu ibi sedere potest.

But Latin could rearrange it for emphasis, for example:

• Haec sella quam illud scamnum commodior est...
• Quia avia ibi diu sedere potest, haec sella commodior est...

The meaning would stay basically the same, though the emphasis might shift.

A few useful observations about the given order:

• Haec sella comes first as the topic.
• commodior est quam illud scamnum gives the comparison.
• quia... then gives the reason.
• sedere potest comes at the end of its clause, which is very typical in Latin prose.

Because commodior is describing sella, not scamnum.

The sentence says:

• This chair is more comfortable than that bench

So the adjective belongs to the first noun, sella. That is why the form is feminine/masculine commodior, not neuter commodius.

If Latin were saying that bench is more comfortable, then with scamnum you would expect:

• illud scamnum commodius est

So agreement depends on the noun being described, not on the noun after quam.