“Bonus” adiectivum est, sed “puer” nomen est.

Because bonus is a word that normally describes a noun, meaning good. In Latin, words like bonus, bona, bonum are adjectives because they modify nouns and agree with them in gender, number, and case.

For example:

• bonus puer = good boy
• bona puella = good girl
• bonum donum = good gift

So in the sentence, Latin is talking about the word bonus and identifying its part of speech.

Because puer is the name of a person or thing—in this case, boy. Words that name people, places, things, or abstract ideas are nouns.

So puer is a noun because it names someone, rather than describing someone.

Est means is. It is the 3rd person singular present tense of esse, to be.

So:

• bonus adiectivum est = bonus is an adjective
• puer nomen est = puer is a noun

Latin often uses est just like English uses is to link one thing to another.

Because there are really two separate statements joined by sed:

• bonus adiectivum est
• puer nomen est

Latin could sometimes leave out the second est if the meaning were clear, but repeating it is completely normal and very clear for learners.

Sed means but.

It joins two contrasting statements:

• bonus is an adjective,
• but puer is a noun.

So sed is a very common coordinating conjunction.

Because bonus is not describing adiectivum.

That is the key point.

In this sentence, bonus is the word being discussed, not an adjective modifying adiectivum. The sentence does not mean the good adjective. It means the word bonus is an adjective.

So there is no agreement relationship between bonus and adiectivum here.

Bonus is the masculine nominative singular form.

It is the basic citation form learners usually meet first. The full set often begins:

• bonus — masculine
• bona — feminine
• bonum — neuter

So when the sentence mentions bonus, it is using that familiar dictionary-style form of the adjective.

Puer is nominative singular.

It is the basic form meaning boy. It belongs to the second declension, but unlike many second-declension masculine nouns, its nominative singular ends in -er, not -us.

For example:

• nominative singular: puer
• genitive singular: puerī

So puer is just the normal dictionary form of that noun.

Because after est, Latin normally puts both the subject and the predicate noun in the nominative.

So in:

• bonus adiectivum est

both bonus and adiectivum are nominative.

And in:

• puer nomen est

both puer and nomen are nominative.

This is similar to English he is a teacher, where teacher renames the subject.

Yes.

Even though adiectivum means adjective, it is itself being used as a noun here: an adjective. Likewise, nomen means noun, and it is also a noun.

So the sentence uses two grammatical terms as ordinary nouns:

• adiectivum = adjective
• nomen = noun

Because Latin does not have an indefinite article.

English says:

• is an adjective
• is a noun

Latin simply says:

• adiectivum est
• nomen est

So learners have to get used to Latin often leaving out words that English requires.

Because the sentence is talking about those words as words.

English does the same thing:

• "Boy" is a noun.
• "Good" is an adjective.

In modern printed Latin, editors often use quotation marks or italics to show that a word is being mentioned rather than used normally in a sentence. Ancient Romans did not use modern quotation marks the way we do, but modern textbooks often do.

It is capitalized simply because it begins the sentence.

That does not mean Latin nouns or adjectives are normally capitalized. In ordinary Latin writing, bonus would usually be lowercase unless it begins a sentence or appears in some special context.

Yes. Latin word order is more flexible than English word order because the endings give a lot of grammatical information.

So these would still be understandable:

• Bonus est adiectivum.
• Puer est nomen.

The textbook order bonus adiectivum est, sed puer nomen est is perfectly good, but it is not the only possible order.

It is just mentioning the word.

That distinction matters:

• In bonus puer, bonus is actually being used as an adjective: good boy.
• In bonus adiectivum est, bonus is not describing anything. The sentence is talking about the word bonus and classifying it.

So this is language about grammar, not ordinary description.