Haec fenestra fragilior est quam illa.

Because fenestra is a feminine singular noun, and the demonstrative adjective must agree with it in gender, number, and case.

• hic = masculine singular nominative
• haec = feminine singular nominative
• hoc = neuter singular nominative

So:

• haec fenestra = this window

Latin agreement is very strict: if the noun is feminine singular nominative, the word meaning this must also be feminine singular nominative.

Fenestra is the nominative singular form, because it is the subject of the sentence.

In this sentence, the thing being described is this window, so Latin uses the nominative:

• haec fenestra = the subject

The verb est means is, so the basic structure is:

• Haec fenestra ... est
• This window ... is

Because the sentence is making a comparison: this window is more fragile than that one.

Latin often forms the comparative adjective by adding -ior (for masculine/feminine) or -ius (for neuter) to the stem.

So from fragilis (fragile) you get:

• fragilior = more fragile (masculine/feminine)
• fragilius = more fragile (neuter)

Since fenestra is feminine, fragilior is the correct form.

Because comparative adjectives in Latin work a little differently from many regular adjectives.

A comparative adjective like fragilior has:

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

So the same form can be masculine or feminine in the nominative singular:

• masculine: fragilior
• feminine: fragilior
• neuter: fragilius

That is why the feminine noun fenestra can still take fragilior.

Quam means than in comparisons.

So:

• fragilior quam illa = more fragile than that one

It links the two things being compared:

1. haec fenestra = this window
2. illa = that one / that window

A very common Latin pattern is:

• comparative adjective + quam
• for example: maior quam, fortior quam, celerior quam

Illa means that one, and the noun fenestra is understood.

So literally the idea is:

• Haec fenestra fragilior est quam illa [fenestra].

Latin often leaves out a noun if it is obvious from context. In English we do the same thing:

• This window is more fragile than that one.

Here illa is feminine singular nominative, matching the omitted fenestra.

Because with comparative + quam, Latin normally puts the second thing compared in the same case as the first if the grammar is parallel.

Here the comparison is between:

• haec fenestra = nominative
• illa [fenestra] = also nominative

So both sides are nominative because both are understood as subjects of the comparison.

In other words, the structure is basically:

• This window is more fragile than that window is.

Since that window would also be a subject, nominative makes sense.

Yes, sometimes Latin can omit est when the meaning is clear, especially in more compressed or literary style. But in a straightforward sentence like this, keeping est is very normal.

So both of these can make sense:

• Haec fenestra fragilior est quam illa.
• Haec fenestra fragilior quam illa.

The version with est is clearer and is especially helpful for learners.

The word order is normal and clear, but Latin word order is more flexible than English word order.

This sentence goes:

• Haec fenestra = this window
• fragilior = more fragile
• est = is
• quam illa = than that one

A very literal ordering is:

• This window more-fragile is than that-one.

Latin uses endings, not just position, to show how words function, so the order can change for emphasis. For example, Latin could rearrange parts of this sentence and still keep the same basic meaning.

They are forms of common Latin demonstratives:

• hic, haec, hoc = this
• ille, illa, illud = that

In this sentence:

• haec modifies fenestra, so it is acting adjectivally: this window
• illa stands by itself, so it is acting pronominally: that one

Latin demonstratives can do both jobs:

• with a noun: haec fenestra = this window
• without a noun: illa = that one

Because Latin usually prefers the comparative adjective form when one exists.

So instead of:

• magis fragilis = more fragile

Latin more naturally says:

• fragilior

Using magis + adjective is possible in some situations, but the ordinary and expected form here is the regular comparative fragilior.

The positive, or basic, form is fragilis, fragile.

Latin adjectives are usually learned in dictionary form as:

• masculine/feminine nominative singular
• neuter nominative singular

So for this adjective:

• fragilis = masculine/feminine
• fragile = neuter

From that adjective, the comparative is formed:

• fragilior (masculine/feminine)
• fragilius (neuter)

Not always. Latin has another way to express comparison: the ablative of comparison.

For example, instead of saying:

• fragilior quam illa

Latin can sometimes say:

• fragilior illa

with illa in the ablative rather than the nominative.

But in your sentence, Latin uses the very clear and common comparative + quam pattern, which is often the easiest one for learners to recognize.