Sub fraxino avia sedet et carmen iucundum cantat.

Because sub can take different cases depending on the meaning:

• sub + ablative = under in the sense of location or position
• sub + accusative = to under / under in the sense of motion toward

Here the person is already sitting there, so it is a location, not movement. That is why Latin uses the ablative:

• sub fraxino = under the ash tree

The dictionary form would be fraxinus.

The basic or dictionary form is fraxinus, meaning ash tree.

In the sentence, it appears as fraxino because it is:

• singular
• ablative
• used after sub to show location

So:

• fraxinus = ash tree
• fraxino = under/by/with the ash tree, depending on context and preposition

Avia is in the nominative singular, which is the case normally used for the subject of a sentence.

Also, both verbs are third person singular:

• sedet = she sits / is sitting
• cantat = she sings / is singing

Since the verbs are singular, they fit a singular subject, and avia is the noun in the nominative form, so it is the subject.

Carmen is in the accusative singular because it is the direct object of cantat.

It answers the question: what is she singing?

• cantat = she sings
• carmen = song

So carmen is the thing being sung, which makes it the direct object.

Iucundum is an adjective meaning pleasant or delightful, and it agrees with carmen.

Since carmen is:

• neuter
• singular
• accusative

the adjective must match it:

• carmen iucundum = a pleasant song

This is called agreement: Latin adjectives agree with the nouns they describe in gender, number, and case.

Because not all neuter nouns are second-declension nouns in -um.

Carmen is a third-declension neuter noun. Its nominative singular is carmen, and its accusative singular is also carmen.

That is normal for neuter nouns:

• nominative singular = accusative singular
• nominative plural = accusative plural

So even though it does not look like bellum, it is still neuter.

Both are present tense, active voice, indicative mood, third person singular.

So:

• sedet = she sits / she is sitting
• cantat = she sings / she is singing

In simple sentences like this, the Latin present tense can often be translated either as a simple present or a present progressive in English.

Because Classical Latin does not have definite or indefinite articles like English the and a/an.

So:

• avia can mean grandmother, the grandmother, or a grandmother
• carmen iucundum can mean a pleasant song or the pleasant song

The exact meaning depends on context.

Latin word order is much more flexible than English word order because Latin uses endings to show grammatical function.

In English, word order does a lot of the work:

• The grandmother sings the song is different from
• The song sings the grandmother

In Latin, the endings already show who is doing what, so writers can move words around more freely.

This sentence places the words as:

• Sub fraxino = prepositional phrase
• avia = subject
• sedet et ... cantat = two verbs
• carmen iucundum = object + adjective

A Latin speaker could rearrange much of this without changing the core meaning.

Yes. Both are grammatically correct.

Latin adjectives can come before or after the noun:

• carmen iucundum
• iucundum carmen

Both mean pleasant song.

Sometimes word order can add emphasis or style, but both are normal Latin.

Et means and, and here it connects the two verbs:

• sedet = sits
• cantat = sings

The same subject, avia, does both actions:

• avia sedet et cantat = the grandmother sits and sings

Putting sub fraxino first gives the setting right away. Latin often places a prepositional phrase or other important detail near the beginning for emphasis or scene-setting.

So the sentence first tells you where the action is happening, and then tells you who is there and what she is doing.

That kind of order is very natural in Latin.