Haec epistula longior est quam illa.

Because epistula is a feminine singular noun.

The demonstrative hic, haec, hoc means this, but its form changes to match the noun’s gender, number, and case.

So here:

• epistula = feminine, singular, nominative
• therefore this letter = haec epistula

A learner often first meets hic as the dictionary form, but hic is only the masculine nominative singular form.

Here it is being used adjectivally, because it directly modifies a noun:

• haec epistula = this letter

Latin demonstratives like hic and ille can be used in two ways:

• adjective use: with a noun
  
  • haec epistula = this letter
• pronoun use: standing by itself
  
  • haec = this woman / this thing / these things, depending on context

In this sentence, the first haec goes with epistula, so it is functioning like an adjective.

Because longior is a comparative adjective, not the ordinary positive form of the adjective.

• longus, longa, longum = long
• longior, longius = longer

Comparative adjectives in Latin are declined differently from ordinary first-and-second-declension adjectives. In the nominative singular, they have:

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

So even though epistula is feminine, the correct form is still:

• epistula longior = a longer letter

Not longa, because longa means long, not longer.

Quam means than in a comparison.

So the pattern is:

• longior ... quam ... = longer ... than ...

In this sentence:

• Haec epistula longior est quam illa.

the first thing is this letter, and the second thing being compared is that one.

A very common Latin comparison pattern is:

• comparative adjective + quam + same-case noun/pronoun

So this sentence literally works like:

• This letter is longer than that one.

Because Latin often omits a noun when it is obvious from context.

Here illa really means something like:

• illa epistula = that letter

But since epistula has already appeared, Latin does not need to repeat it. This is very natural and common.

So:

• quam illa = than that one
• more fully: quam illa epistula

English does the same thing:

• This letter is longer than that one.

We do not have to repeat letter either.

Here illa is nominative feminine singular.

Why nominative? Because after quam, Latin normally puts the second item of the comparison in the same case as the first item.

The first item is:

• haec epistula — nominative, because it is the subject

So the second item is also nominative:

• illa — nominative feminine singular, with epistula understood

So the comparison is:

• haec epistula
• quam illa (epistula)

both nominative.

Est is the verb is.

Latin often uses forms of esse just as English uses to be:

• epistula longior est = the letter is longer

In some contexts, especially in poetry or very compressed style, Latin can omit est when it is easily understood. But in a normal straightforward sentence like this, including est is completely standard and natural.

So here est is simply the linking verb connecting:

• haec epistula
  and
• longior

Latin word order is more flexible than English word order, because the endings show how the words function.

So this sentence could be rearranged in several ways without changing the basic meaning, for example:

• Haec epistula longior est quam illa.
• Haec epistula quam illa longior est.
• Longior est haec epistula quam illa.

That said, not every order is equally natural in every context. The given version is a clear, ordinary way to say it.

A useful rule for learners is:

• English depends heavily on word order.
• Latin depends much more on inflection.

Yes. Latin can often use the ablative of comparison instead of quam.

So alongside:

• Haec epistula longior est quam illa.

you could also have:

• Haec epistula longior est illā epistulā.

That literally means something like:

• This letter is longer by comparison with that letter

In this version:

• illā epistulā is ablative

Both constructions are common, and learners should recognize both.

A simple contrast:

• quam + same case
• ablative without quam

Yes. Longior agrees with epistula in:

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

Even though the nominative singular feminine comparative form looks the same as the masculine, it is still feminine here because it agrees with a feminine noun.

So:

• epistula longior = feminine singular nominative

This is a good reminder that agreement is about grammar, not just about whether a form looks obviously feminine to an English-speaking learner.

In a common restored classical pronunciation:

• haec is roughly like hike
• quam is roughly kwahm

And the whole sentence might be pronounced approximately:

• haik eh-PIST-oo-lah LONG-gee-or est kwahm IL-lah

A few helpful points:

• ae often sounds like the vowel in eye
• qu sounds like kw
• g is always hard, as in go
• ll is pronounced as two clear l sounds

If you are using an ecclesiastical pronunciation system, some details may differ, but the grammar stays the same.