Haec ianua firmior est quam illa.

Because ianua is a feminine noun in Latin.

The demonstrative meaning this has to agree with the noun it describes in:

• gender
• number
• case

So with a feminine singular nominative noun like ianua, the correct form is haec.

A quick comparison:

• hic = this, masculine singular nominative
• haec = this, feminine singular nominative
• hoc = this, neuter singular nominative/accusative

So haec ianua means this door.

Both haec and ianua are in the nominative singular.

That is because haec ianua is the subject of the sentence: it is the thing being described as stronger.

Breakdown:

• haec = nominative singular feminine
• ianua = nominative singular feminine
• firmior est = is stronger

So literally the structure is:

• This door = subject
• is stronger = predicate

Firmior is the comparative form of firmus, meaning firm, strong, or sturdy.

Latin usually forms the comparative by adding:

• -ior for masculine/feminine
• -ius for neuter

So:

• firmus = strong
• firmior = stronger
• firmius = stronger, neuter form

In this sentence, firmior agrees with ianua, which is feminine singular nominative. For comparatives, the nominative singular masculine and feminine are the same form: firmior.

Because comparative adjectives do not use first/second-declension endings like firmus, firma, firmum.

Instead, comparatives follow a different pattern:

• masculine nominative singular: firmior
• feminine nominative singular: firmior
• neuter nominative singular: firmius

So even though ianua is feminine, the correct nominative singular comparative is still firmior, not firmiora.

Firmiora would be a neuter plural form, so it would not fit here.

Quam means than in a comparison.

So:

• firmior est quam illa = is stronger than that one

It introduces the second part of the comparison.

The pattern is:

• comparative adjective + quam + thing compared

For example:

• maior quam = bigger than
• celerior quam = faster than
• firmior quam = stronger than

Yes, the full idea is basically than that door, and Latin could say quam illa ianua.

But Latin often leaves out a noun when it is obvious from context. This is very common.

So here:

• illa = that one
• understood noun = ianua

In other words:

• Haec ianua firmior est quam illa
  = This door is stronger than that one = literally This door is stronger than that [door]

Here illa is best understood as nominative singular feminine.

That is because it stands for illa ianua, and with quam the compared word is often in the same case as the first item.

So the comparison is:

• haec ianua = nominative
• illa understood as illa ianua = also nominative

This is why the sentence works as:

• This door is stronger than that one

Yes. Latin can often make comparisons in two ways:

1. comparative + quam
2. comparative + ablative

So alongside Haec ianua firmior est quam illa, Latin could also use an ablative form meaning:

• This door is stronger than that one

With the ablative construction, the form of illa would change to illā.

For a learner, the important point is:

• quam + same case is one normal way
• ablative without quam is another normal way

Est means is, and it links the subject to the description:

• haec ianua = this door
• firmior = stronger
• est = is

So literally:

• This door is stronger than that one

Latin sometimes can omit forms of to be, especially in poetry or very compressed style, but in ordinary prose est is perfectly normal and expected here.

They are both demonstratives, but they point differently.

• haec = this; something nearer, more immediate, or more present to the speaker
• illa = that; something more distant or less immediate

So the sentence contrasts:

• this door with
• that one

This is very similar to English this versus that.

The word order is natural, but Latin word order is more flexible than English word order.

This sentence is arranged as:

• Haec ianua = this door
• firmior est = is stronger
• quam illa = than that one

Latin could rearrange the words somewhat and still keep the same basic meaning, because the endings show the grammar.

However, this order is clear and straightforward for learners:

• subject first
• description next
• comparison last

Yes. Firmior agrees with ianua in:

• number: singular
• case: nominative
• gender: feminine

The only reason it may not look obviously feminine is that comparative adjectives have the same nominative singular form for masculine and feminine:

• masculine: firmior
• feminine: firmior

So even though the form does not change between masculine and feminine here, it is still agreeing correctly with ianua.

Ianua is a regular first-declension feminine noun meaning door.

Its nominative singular ends in -a, which is very typical for first-declension feminine nouns.

What may feel unusual to an English speaker is not ianua, but the words around it:

• haec
• illa
• firmior

Those are pronouns and a comparative adjective, so they follow patterns that are less simple than a basic noun like ianua.