In sententia “puella laeta est” adiectivum cum nomine convenit.

Because in with the ablative often means in or within in a location or setting. Here in sententia means in the sentence.

So:

• in + ablative = in, on, within
• sententia is the ablative singular of sententia

If Latin meant motion into something, it would usually use in + accusative instead.

Because the preposition cum takes the ablative case.

So:

• cum = with
• nomen, nominis = noun
• nomine = ablative singular, meaning with the noun

This is a very common pattern in Latin: cum + ablative.

Here convenit means agrees.

The verb is from convenire, which can mean things like come together, fit, or agree, depending on context. In grammar, convenire cum is commonly used for to agree with.

So adiectivum cum nomine convenit means:

• the adjective agrees with the noun

Grammatically, convenit is:

• 3rd person singular
• present tense
• active voice

It is singular because its subject is adiectivum: one adjective.

In Latin, an adjective must match the noun it describes in:

• gender
• number
• case

In puella laeta est:

• puella is feminine, singular, nominative
• laeta is also feminine, singular, nominative

That matching is what agreement means.

This is much more important in Latin than in English, because English adjectives usually do not change form. In English we say happy girl and happy girls; happy stays the same. In Latin, the adjective changes its ending.

Because it has to agree with puella.

The noun puella is:

• feminine
• singular
• nominative

So the adjective must also be:

• feminine
• singular
• nominative

The adjective laetus, laeta, laetum means happy. Its forms are:

• laetus = masculine nominative singular
• laeta = feminine nominative singular
• laetum = neuter nominative singular

Since puella is feminine, Latin uses laeta.

They are both nominative singular.

Puella is nominative because it is the subject of est.

Laeta is also nominative because it is a predicate adjective with the linking verb est. In Latin, predicate adjectives usually stand in the nominative and agree with the subject.

So the structure is:

• puella = the subject
• laeta = a predicate adjective describing the subject
• est = is

Because agreement in Latin does not depend on whether the adjective is used:

• attributively: directly with the noun, as in laeta puella = the happy girl
• predicatively: with a linking verb, as in puella laeta est = the girl is happy

In both uses, the adjective agrees with the noun in gender, number, and case.

So laeta agrees with puella whether it stands next to it or is separated from it by other words.

It is a predicate adjective.

A predicate adjective is an adjective used with a linking verb such as est to describe the subject.

Compare:

• laeta puella = the happy girl
  
  • adjective used directly with the noun
• puella laeta est = the girl is happy
  
  • adjective used as part of the predicate

Latin allows both patterns, but in either case the adjective still agrees with the noun.

Because its subject is puella, which is singular.

Est is the 3rd person singular present form of esse, meaning to be:

• sum = I am
• es = you are
• est = he/she/it is

Since puella means girl in the singular, Latin uses est, not sunt.

If the subject were plural, you would get:

• puellae laetae sunt = the girls are happy

Yes. Latin word order is more flexible than English because the endings show the grammatical relationships.

So these can all express roughly the same basic idea:

• puella laeta est
• laeta puella est
• puella est laeta

However, word order can affect emphasis or style. The textbook order puella laeta est is a simple, clear arrangement for beginners.

The dictionary form is usually given as:

• laetus, laeta, laetum

This shows that it is a 1st/2nd-declension adjective with masculine, feminine, and neuter nominative singular forms.

So laeta is not the basic form by itself; it is one form of the adjective laetus.

Because here adiectivum is being used as a noun meaning adjective.

Latin grammatical terms are often neuter when used this way. You can think of adiectivum as a shortened form related to nomen adiectivum, meaning adjective.

It is singular because the sentence is talking about the adjective in the example sentence, namely laeta.

Not necessarily. Agreement means they match in gender, number, and case, not that they must look identical.

Sometimes the endings are the same, but sometimes they are different because the noun and adjective belong to different declension patterns.

For example, an adjective can still agree with a noun even if the forms do not look alike, as long as both are, for example:

• feminine
• singular
• nominative

So learners should focus on grammatical features, not just on whether the endings happen to match exactly.