Frater et soror libros similes legunt, sed de rebus diversis inter se disputant.

Because frater et soror is a compound subject: brother and sister together make more than one person, so the verb is plural.

• frater = brother (singular)
• soror = sister (singular)
• frater et soror = brother and sister = they

So Latin uses:

• legunt = they read
• disputant = they discuss / argue

They are in the nominative singular, because they are the subject of the sentence.

You can think of the basic structure like this:

• Frater et soror = the ones doing the actions
• legunt = read
• disputant = discuss

In Latin, the subject is commonly put in the nominative case.

Libros is accusative plural of liber (book). It is in the accusative because it is the direct object of legunt.

So:

• legunt = they read
• What do they read? libros = books

That is why libros is accusative.

Similes agrees with libros.

Since libros is:

• masculine
• plural
• accusative

the adjective describing it must match those features.

So:

• libros = masculine plural accusative
• similes = masculine/feminine plural accusative of similis

Similia would be neuter plural, so it would not match libros.

It is an adjective modifying libros.

So:

• libros similes = similar books

A learner may expect something more like the same books, but similes means similar, not necessarily identical.

Because the preposition de takes the ablative case.

Here:

• rebus is the ablative plural of res (thing, matter)
• diversis is the ablative plural of diversus (different, diverse)

So:

• de rebus diversis = about different things / matters

The adjective must agree with the noun:

• rebus = ablative plural
• diversis = ablative plural

Because diversis has to agree with rebus, and rebus is ablative plural.

If it were diversas, that would be accusative plural feminine, which would not fit after de.

So the pattern is:

• de
  • ablative
• rebus = ablative plural
• diversis = ablative plural

Inter se means with each other, among themselves, or between themselves.

The two parts are:

• inter = between / among
• se = themselves (a reflexive pronoun)

Together, they show that the action happens mutually between the people in the subject.

So:

• inter se disputant = they discuss/argue with each other

Because se is the reflexive pronoun. It refers back to the subject of the clause.

Here the subject is frater et soror, so se means themselves.

If Latin used eos or eas, that would usually refer to some other people, not back to the subject itself.

So inter se clearly means that the brother and sister are interacting with each other.

Both are present tense, third person plural, active, indicative.

So:

• legunt = they read / are reading
• disputant = they discuss / are discussing / argue

The ending -unt is a strong clue for third person plural in many present-tense verb forms.

Their dictionary forms are:

• legunt → lego, legere = to read
• disputant → disputo, disputare = to discuss, debate, argue

A learner may notice that legunt comes from a third-conjugation verb, while disputant comes from a first-conjugation verb.

Because Latin does not have articles like English the and a/an.

So:

• frater can mean a brother or the brother
• libros can mean books, the books, or sometimes some books

The exact sense depends on context.

Latin word order is more flexible than English word order, because the case endings show how the words function.

So this sentence could be rearranged in various ways without changing the basic meaning, for example:

• Frater et soror libros similes legunt
• Libros similes frater et soror legunt

Both still mean that the brother and sister read similar books.

However, word order can affect emphasis. The given order is a very natural, straightforward one.

It can mean either, depending on context.

In classroom Latin, disputare often means:

• to discuss
• to debate
• sometimes to argue

So inter se disputant could be understood as:

• they discuss things with each other
• they debate with each other
• they argue with each other

The exact tone depends on the context, not just the grammar.