In bibliotheca muri veteres sunt, sed tectum tutum manet.

Because in with the ablative usually means in or inside a place where something already is.

So:

• in bibliotheca = in the library

If Latin wanted to express movement into the library, it would normally use in with the accusative:

• in bibliothecam = into the library

That is a very common distinction in Latin:

• in + ablative = location
• in + accusative = motion toward / into

Because muri is the subject of sunt.

The first clause is:

• muri veteres sunt = the walls are old

Here:

• muri = walls
• nominative plural = subject
• sunt = are

A native English speaker may expect word order to tell them the subject, but in Latin the case ending does that job. The ending -i in muri shows nominative plural for a second-declension masculine noun.

Because it agrees with muri.

Latin adjectives agree with the nouns they describe in:

• gender
• number
• case

Since muri is:

• masculine
• plural
• nominative

veteres must also be masculine plural nominative.

So:

• muri veteres = old walls

Even though the adjective form veteres can also be used in other genders in the plural, its exact role here is clear because it agrees with muri.

Because the two verbs mean different things.

• sunt = are
• manet = remains / stays

So the sentence is not just saying two simple facts of the same kind. It contrasts them:

• the walls are old
• but the roof remains safe

That gives manet a sense of continued condition: despite the old walls, the roof is still sound or still safe.

Also, the verb forms match their subjects:

• muri is plural, so sunt is plural
• tectum is singular, so manet is singular

They are nominative singular neuter.

• tectum = roof
• tutum = safe or secure

Here tectum is the subject of manet, and tutum agrees with it.

Because tectum is neuter singular nominative, tutum must also be neuter singular nominative.

So:

• tectum tutum manet = the roof remains safe

In this sentence, it is best understood as a predicate adjective with manet.

That means the idea is:

• the roof remains safe

not merely:

• the safe roof remains

Latin often allows adjective placement that could look ambiguous to an English learner, but the verb manet strongly encourages the predicate sense here. The adjective tells us the condition in which the roof remains.

So the structure is roughly:

• subject: tectum
• predicate adjective: tutum
• verb: manet

Because Latin word order is much freer than English word order.

English depends heavily on word order:

• The old walls are in the library

Latin depends much more on endings:

• muri tells you the subject
• bibliotheca after in tells you location
• veteres agrees with muri

So Latin can place words in different positions for emphasis, rhythm, or style. In this sentence:

• In bibliotheca sets the scene first
• muri veteres sunt gives the first statement
• sed tectum tutum manet gives the contrast

A different order could still be grammatical, for example:

• Muri veteres in bibliotheca sunt, sed tectum tutum manet

But the original order is perfectly natural.

Sed means but, and it introduces a contrast.

The sentence sets up two ideas:

• the walls are old
• but the roof remains safe

So sed signals that the second clause contrasts with what you might expect from the first. Old walls might suggest a building in poor condition, but the roof is still sound.

Using est would change the nuance.

• tectum tutum est = the roof is safe
• tectum tutum manet = the roof remains safe

Manet adds the idea of continuing in that state. It implies stability or persistence. That makes it a more expressive choice here, especially after the contrast with the old walls.

Bibliotheca is a first-declension noun, borrowed from Greek, and it means library.

In this sentence it appears as:

• bibliotheca = ablative singular after in

A learner may notice that this form looks exactly like the nominative singular. That is normal for many first-declension nouns:

• nominative singular: bibliotheca
• ablative singular: bibliotheca

So only the context tells you which one it is here. Since it follows in meaning location, it is ablative.

Both murus and paries can refer to a wall, but they are not always used in exactly the same way.

• murus often suggests a wall as a structure, boundary, or substantial wall
• paries often means the wall of a room or building interior

In many learning sentences, muri is used simply as a common word for walls. So there is nothing wrong with it here. It is just one standard Latin noun the learner is expected to know.

Because vetus is an irregular-looking third-declension adjective.

Its forms include:

• nominative singular: vetus = old
• nominative plural masculine/feminine: veteres
• nominative singular neuter: vetus
• nominative plural neuter: vetera

So:

• muri veteres = old walls

This is worth memorizing, because vetus does not form its plural the way a first- or second-declension adjective would.