La unua paĝo estas skribita per krajono, sed la dua estas skribita per plumo.

Because la is the definite article, like the in English.

In Esperanto, ordinal numbers such as unua often appear with la when you mean a specific item in an ordered set:

• la unua paĝo = the first page
• la dua ĉapitro = the second chapter

Without la, unua paĝo would sound more like a first page or first page in a less specific sense.

Because unua is an adjective.

In Esperanto:

• -a = adjective ending
• -o = noun ending
• -e = adverb ending

So:

• unu = one
• unua = first

Ordinal numbers are formed with -a:

• unua = first
• dua = second
• tria = third

They behave like adjectives, so they describe nouns:

• unua paĝo
• dua libro
• tria tago

The -o marks a noun.

Esperanto uses very regular endings:

• paĝo = page
• krajono = pencil
• plumo = pen

All of these are nouns, so they end in -o.

This is one of the most useful things in Esperanto: you can often tell a word’s job just by its ending.

Yes. Adjectives agree with the nouns they describe.

Here:

• unua is an adjective
• paĝo is a singular noun

Since the noun is singular and not accusative, the adjective also appears in its basic singular form:

• la unua paĝo

If the noun were plural, both would change:

• la unuaj paĝoj = the first pages

If accusative were needed, both would take -n:

• Mi vidas la unuan paĝon = I see the first page

So adjective agreement is very regular in Esperanto.

Because the noun paĝo is understood from the first part, so Esperanto leaves it out.

This is similar to English:

• the first page is written with pencil, but the second is written with pen

In Esperanto, this is very natural:

• la unua paĝo ... sed la dua ...

Here la dua means the second one, with paĝo understood.

Because estas skribita is a passive form.

• skribi = to write
• skribas = writes / is writing
• estas skribita = is written

So:

• Li skribas la paĝon = He writes the page.
• La paĝo estas skribita = The page is written.

In your sentence, the focus is on the pages and what writing instrument was used, not on who wrote them. That is why the passive is used.

Skribita is the past passive participle of skribi.

Very roughly:

• root: skrib- = write
• -it- = completed passive participle
• -a = adjective ending

So skribita literally means something like written.

Because participles can act like adjectives, they combine with esti:

• estas skribita = is written
• estis skribita = was written
• estos skribita = will be written

Per means by means of, using, or with in the sense of an instrument.

So:

• per krajono = with a pencil / by means of a pencil
• per plumo = with a pen

This is the normal preposition for tools or means:

• Mi tranĉas per tranĉilo. = I cut with a knife.
• Ŝi vojaĝis per trajno. = She traveled by train.

A native English speaker often wants to map English with directly, but Esperanto often uses different prepositions depending on the meaning. Here per is the right one because it expresses the instrument used.

In this sentence:

• krajono = pencil
• plumo = pen

Historically, plumo originally meant feather, and from that it came to mean pen, like older writing quills. In modern Esperanto, plumo is the normal word for pen.

So the contrast in the sentence is simply between two writing instruments.

Because there is no direct object here.

The main structure is:

• La unua paĝo = subject
• estas skribita = verb phrase
• per krajono = prepositional phrase

And then again:

• la dua = subject
• estas skribita = verb phrase
• per plumo = prepositional phrase

The accusative -n is usually used for direct objects, but here the pages are not direct objects; they are the subjects of passive clauses.

Also, nouns after prepositions like per normally do not take -n, unless there is some special reason unrelated to the preposition.

Sed means but.

It connects two contrasting statements:

• the first page is written one way
• the second is written another way

The comma before sed is normal and helpful, just as in English before but when joining two full clauses.

So the sentence is neatly divided into two parallel parts:

• La unua paĝo estas skribita per krajono
• sed la dua estas skribita per plumo

Yes, but it means something different.

• estas skribita = is written / has been written, with emphasis on the completed written state
• estas skribata = is being written, with emphasis on the ongoing action

So:

• La unua paĝo estas skribita per krajono suggests the page is in a written state, and the writing was done in pencil.
• La unua paĝo estas skribata per krajono suggests the page is currently being written in pencil.

In your sentence, skribita is the better choice because it describes how the page is written, not the process happening right now.

Yes. It has a very clear parallel structure, which is common in Esperanto and makes sentences easy to understand.

Pattern:

• La unua paĝo estas skribita per krajono
• sed la dua estas skribita per plumo

The second half repeats the same structure but leaves out paĝo because it is already understood. This kind of symmetry is very natural in Esperanto and often makes long sentences easier to follow.