Scriba epistulam longiorem conscripsit, et pater epistulam subscriptam ad amicum misit.

Because scriba is a first-declension noun, and some first-declension nouns referring to occupations or people are masculine.

Common examples:

• scriba = scribe, clerk
• agricola = farmer
• poēta = poet
• nauta = sailor

So here scriba is masculine nominative singular and is the subject of conscripsit.

Latin usually shows that by case endings, not mainly by word order.

In this sentence:

• scriba = nominative singular → subject of conscripsit
• pater = nominative singular → subject of misit
• epistulam = accusative singular → direct object
• amicum = accusative singular after ad

So even if the order changed, the endings would still tell you who is doing the action and what is receiving it.

Because longiorem is the comparative form of longus.

• longa = long
• longior = longer
• longiorem = longer, in the feminine accusative singular form

It agrees with epistulam, which is also feminine accusative singular.

So:

• epistulam longam = a long letter
• epistulam longiorem = a longer letter or sometimes a rather long letter, depending on context

Not always.

A Latin comparative can:

• make an actual comparison, even if the other thing is not stated
• sometimes have a softer sense like rather long or somewhat long

So epistulam longiorem may mean:

• a longer letter
• or, in some contexts, a fairly long letter

Grammar alone does not tell you exactly what it is being compared with.

Conscripsit is the perfect active indicative, third-person singular, of conscribere.

Basic idea:

• scribere = to write
• conscribere = to write out, compose, draw up

So conscripsit means wrote, composed, or drew up.

The perfect tense here shows a completed past action.

Subscriptam is a perfect passive participle from subscribere.

It agrees with epistulam in:

• gender: feminine
• number: singular
• case: accusative

So epistulam subscriptam means the signed letter or the letter having been signed.

It describes the letter as already in a signed state when the father sent it.

Because ad takes the accusative and commonly expresses movement to or toward someone or something.

So:

• ad amicum = to a friend

With verbs of sending, Latin often uses ad + accusative to show destination.

A dative such as amico can be used as an indirect object in Latin, but mittere ad aliquem is a very normal and common expression.

Latin often repeats a noun where English would prefer it or would mention the noun only once.

Reasons for repetition can include:

• clarity
• emphasis
• balance between the two clauses

So repeating epistulam is perfectly natural Latin.

Also, grammar alone does not force the two letters to be the same letter. The sentence could mean the same letter or two different letters, depending on context.

Because Classical Latin has no articles.

So epistulam can mean:

• a letter
• the letter

You decide from context which is more natural in translation.

Yes. It is a very natural Latin word order.

Each clause is basically:

• subject
• object + modifier
• verb

So:

• Scriba epistulam longiorem conscripsit
• et pater epistulam subscriptam ad amicum misit

Latin word order is flexible, though, because the endings do most of the grammatical work. The same idea could be rearranged in other ways without changing the basic meaning.

Yes.

Both are:

• third-person singular
• perfect
• active
• indicative

So both mean a completed action in the past:

• conscripsit = he wrote / composed
• misit = he sent

That gives the sentence a straightforward narrative feel: one person wrote, and another person sent.

Because pater is the subject of misit, so it must be in the nominative case.

• pater = nominative singular = father as subject
• patrem = accusative singular = father as object

Here the father is doing the sending, so pater is correct.