Avus codicem vetustum libenter legit.

Because avus is in the nominative singular, the case normally used for the subject.

• avus = grandfather
• nominative singular ending: -us

So avus is the one doing the action.

Because codicem is in the accusative singular, which is the case normally used for a direct object.

• nominative: codex = book, codex
• accusative: codicem

So codicem is the thing being read.

Vetustum is an adjective meaning old or ancient, and it must agree with the noun it describes.

It describes codicem, so it matches it in:

• gender: masculine
• number: singular
• case: accusative

That is why we get codicem vetustum = an old book or the old book.

Libenter is an adverb. It modifies the verb legit and tells us how the action is done.

So:

• legit = reads
• libenter legit = reads gladly or likes to read

It does not change form, because adverbs do not agree with nouns the way adjectives do.

Latin word order is much more flexible than English word order because the endings show each word’s role.

Putting the verb at the end is very common in Latin, especially in simple textbook sentences and in much prose.

So this order is natural:

• Avus codicem vetustum libenter legit.

But Latin could also rearrange the words for emphasis without changing the basic meaning.

Yes, but mostly for emphasis, not for the basic grammar.

Because the case endings already tell us who is doing what, Latin does not depend on word order as heavily as English does.

For example, these could mean essentially the same thing:

• Avus codicem vetustum libenter legit.
• Codicem vetustum avus libenter legit.
• Libenter avus codicem vetustum legit.

The most neutral difference is usually emphasis:

• first word often gets attention
• last word, especially the verb, can also be prominent

If your translation shows a present meaning, then legit is the present active indicative, third person singular of legere:

• lego = I read
• legis = you read
• legit = he/she reads

One important detail: in texts without macrons, legit can also look like the perfect lēgit = he/she read or has read. Usually the context tells you which meaning is intended.

Latin does not have articles like English the, a, or an.

So avus can mean:

• grandfather
• a grandfather
• the grandfather

And codicem vetustum can mean:

• an old book
• the old book

The context decides which English article is best in translation.

They are:

• avus, avi = grandfather
• codex, codicis = book, codex
• vetustus, vetusta, vetustum = old, ancient
• libenter = gladly, willingly
• lego, legere, legi, lectum = read

A learner often needs the dictionary form because the word in the sentence may be an inflected form, not the form listed in a vocabulary list.

Because codex is a third-declension noun, and third-declension nouns often change their stem between the nominative and the other cases.

Here the forms are:

• nominative singular: codex
• genitive singular: codicis
• stem: codic-
• accusative singular: codicem

So codicem is built from the stem codic- plus the accusative singular ending -em.

The basic pattern is:

• subject: avus
• direct object: codicem vetustum
• adverb: libenter
• verb: legit

So grammatically it is:

Subject + Object + Adverb + Verb

That is a very common and useful pattern to recognize in Latin.