In horto rosae et lilia florent, et avia dicit hortum hodie pulchriorem esse quam heri.

Because in takes the ablative when it means in or on in the sense of location.

• in horto = in the garden
• If it meant motion into the garden, Latin would usually use the accusative: in hortum

So this is the normal place where construction.

They belong to different declensions, so their nominative plural endings are different.

• rosa, rosae is a 1st-declension noun, so nominative plural is rosae
• lilium, lilii is a 2nd-declension neuter noun, so nominative plural is lilia

A common beginner point: neuter plural nouns often end in -a, so lilia is plural even though it may look singular at first.

We know from both form and sense.

1. rosae and lilia are in nominative forms here
2. florent is a plural verb, so it needs a plural subject
3. The meaning fits naturally: the roses and lilies are doing the blooming

Latin word order is flexible, so you do not rely only on position the way you often do in English.

Because its subject is compound: rosae et lilia = roses and lilies.

A compound subject takes a plural verb, so:

• rosa floret = the rose blooms
• rosae florent = the roses bloom
• rosae et lilia florent = roses and lilies bloom

Because avia is the subject of dicit.

• avia = grandmother, nominative singular
• dicit = says

So avia dicit means grandmother says. Latin marks the subject by case, not mainly by word order.

Because Latin usually does not use a normal English-style that-clause after a verb of saying, thinking, knowing, and so on. Instead, it often uses an indirect statement construction:

• accusative + infinitive

So instead of something like Grandmother says that the garden is more beautiful, Latin says:

• avia dicit hortum ... pulchriorem esse

This is one of the most important Latin constructions to learn.

Because the same noun has two different jobs in the sentence.

• in horto: horto is ablative after in, showing location
• hortum ... esse: hortum is accusative because it is the subject of an indirect statement

So the case changes because the grammatical role changes.

In Latin indirect statement, the logical subject of the infinitive goes into the accusative.

Direct statement:

• Hortus hodie pulchrior est quam heri.
• The garden is more beautiful today than yesterday.

Indirect statement after dicit:

• Avia dicit hortum hodie pulchriorem esse quam heri.
• Grandmother says the garden to be more beautiful today than yesterday

That sounds unnatural in English, so we translate it with that the garden is..., but Latin uses hortum in the accusative.

Because indirect statement uses the infinitive, not a finite verb.

• Direct: hortus pulchrior est
• Indirect after dicit: hortum pulchriorem esse

So esse is simply the infinitive to be. This goes together with the accusative subject hortum.

Because it is the comparative form of pulcher and it agrees with hortum.

• Positive: pulcher = beautiful
• Comparative: pulchrior / pulchrius = more beautiful

Here it must match hortum, which is:

• masculine
• singular
• accusative

So the correct form is pulchriorem.

A useful comparison:

• nominative masculine singular: pulchrior
• accusative masculine singular: pulchriorem
• neuter nominative/accusative singular: pulchrius

Because heri is an adverb, not a noun.

• quam = than
• heri = yesterday
• hodie = today

So the sentence compares the garden’s beauty today with its beauty yesterday:

• hodie pulchriorem ... quam heri = more beautiful today than yesterday

Since heri is already an adverb, it does not need a case ending.

Hodie is an adverb meaning today, and it modifies the idea is more beautiful.

So the sense is:

• the garden is more beautiful today than yesterday

In other words, the comparison is being made across time: today versus yesterday.

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

For example, the second half could also be arranged as:

• et avia dicit hodie hortum pulchriorem esse quam heri

or even in other variations.

That said, the given order is perfectly natural. Latin word order is often used for emphasis, rhythm, or style rather than strict grammatical necessity.