Regina in die festo coronam pulchram gerit.

Because regina is in the nominative singular, which is the case normally used for the subject.

• regina = queen
• nominative singular of a first-declension noun

So regina is the person doing the action: she is the one who wears.

Because coronam is the direct object, so it must be in the accusative singular.

• corona = nominative singular, crown
• coronam = accusative singular, crown as the thing being worn

In this sentence, the queen is wearing the crown, so crown is the object of gerit, and Latin marks that with -am.

Because pulchram is an adjective describing coronam, and Latin adjectives must agree with the nouns they describe in:

• gender
• number
• case

Here:

• coronam is feminine
• singular
• accusative

So the adjective must also be feminine, singular, accusative:

• pulchra = beautiful
• pulchram = beautiful, matching coronam

Gerit is:

• 3rd person singular
• present tense
• active voice
• indicative mood

It comes from the verb gero, gerere, which can mean carry, bear, wear, or carry on, depending on context.

Here it means wears.

So regina ... gerit = the queen wears ...

Latin in can be translated in different ways depending on context.

With time expressions, in + ablative can often mean something like:

• on
• during
• in

So in die festo is naturally translated as on a festival day or on a feast day, even though the Latin word is in.

This is normal. Languages do not always match prepositions word-for-word.

Because in takes different cases depending on its meaning:

• in + ablative = in / on something, showing location or state
• in + accusative = into / onto something, showing motion toward

Here there is no movement into anything. The phrase is just setting the time: on a feast day.

So Latin uses the ablative:

• die = ablative singular of dies
• festo = ablative singular agreeing with die

It is ablative singular.

• dies = day
• die = ablative singular
• festo = ablative singular of festus, agreeing with die

Together they mean on a feast day or on a festival day.

Because festo must agree with die.

The noun dies is usually masculine in this kind of usage, so the adjective must also be:

• masculine
• singular
• ablative

That gives festo.

So:

• dies festus = a feast day
• in die festo = on a feast day

Yes. Dies belongs to the fifth declension, while regina and corona belong to the first declension.

That is why their endings are different.

For example:

• regina → nominative singular, first declension
• coronam → accusative singular, first declension
• dies → nominative singular, fifth declension
• die → ablative singular, fifth declension

Latin has several declensions, and each has its own pattern of endings.

Because Latin word order is much freer than English word order.

Both of these can mean the same thing:

• coronam pulchram
• pulchram coronam

The endings show the grammatical relationships, so Latin does not depend on word order as much as English does.

That said, different word orders can create slightly different emphasis or style.

Yes. Latin often allows several word orders without changing the basic meaning, because the endings already show who is doing what.

For example, these would still mean essentially the same thing:

• Regina coronam pulchram in die festo gerit.
• In die festo regina coronam pulchram gerit.
• Coronam pulchram regina in die festo gerit.

The most natural or elegant order depends on style and emphasis.

Because Classical Latin has no articles.

English uses:

• the queen
• a crown

Latin usually just says:

• regina
• corona / coronam

Whether you translate them as the or a depends on the context.

So regina can mean:

• the queen
• a queen

and coronam can mean:

• a crown
• the crown

No. Gero is a flexible verb with several meanings, including:

• carry
• bear
• wear
• manage
• wage (as in bellum gerere, to wage war)

In this sentence, because the object is coronam, the meaning wear makes the most sense.

So the exact English translation of gerit depends on what is being gerit.

Yes. Latin often uses the ablative of time when without a preposition.

So both of these are possible in Latin:

• die festo
• in die festo

Both can mean on a feast day.

A learner should simply recognize that Latin can express time either with a bare ablative or with in + ablative, depending on style and author.