Cum tibia canit et cithara simul pulsatur, sonitus totius atrii mutatur.

Here cum means when and introduces a straightforward time clause. In that use, Latin often uses the indicative, so canit and pulsatur are exactly what you would expect.

A cum clause with the subjunctive usually has a different feel, such as since, although, or a more descriptive background circumstance.

Not necessarily in the narrow English sense of sing. The verb canere is broader than English to sing and can mean to sound, to play, or to make music, especially with voices and wind instruments.

So tibia canit is a natural Latin way to say that the flute is sounding or playing.

Because tibia is the subject of canit. The sentence is structured as the flute plays/sounds, not someone plays with a flute.

If Latin wanted to say someone plays with a flute, then the flute could appear in the ablative of means: tibiā. Here, though, the instrument itself is doing the action grammatically.

Latin often uses different verbs for different kinds of instruments. A wind instrument like a tibia can canere; a stringed instrument like a cithara is often described with pulsare, literally to strike or pluck.

So the difference is not random. It reflects the natural Latin way of talking about how those instruments are played.

Here it is nominative singular, because it is the subject of pulsatur: the lyre is played.

This can be confusing because a first-declension noun like cithara has -a in the nominative singular, while the ablative singular would be -ā. If macrons are not written, the two forms look the same, so you have to use the syntax to tell them apart.

Et simply links the two actions: the flute plays and the lyre is played. Simul adds the idea at the same time.

So simul is not redundant. It emphasizes that the two musical sounds happen together.

Sonitus here is nominative singular, and it is the subject of mutatur.

It is a fourth-declension noun, which can be tricky because forms like sonitus may not look unusual at first glance. The singular verb mutatur shows that sonitus is functioning as a singular subject.

Because they depend on sonitus: the sound of the whole atrium. Latin often uses the genitive after a noun like sonitus to show what the sound belongs to or comes from.

So:

• atrii = of the atrium
• totius = of the whole, agreeing with atrii

Because totus, tota, totum is one of the adjectives with an irregular genitive singular: totius. It belongs to the group often called the -ius adjectives.

So totius atrii means of the whole atrium.

Latin muto is basically a transitive verb: to change something. So when Latin wants the idea is changed or undergoes change, it naturally uses the passive: mutatur.

In English, we often say the sound changes with an intransitive active verb. Latin often expresses that same idea with a passive form.

Latin does not have to say. The passive can be used when the agent is unknown, obvious, or simply unimportant.

So cithara pulsatur means the lyre is being played, without naming the player. If Latin wanted to name the agent, it could use ab plus the ablative.

Because Latin has no articles. It does not have separate words that exactly match English the and a/an.

Whether you translate tibia as the flute or a flute depends on the context.

Latin word order is much freer because the endings show how the words function. English depends much more on position.

In this sentence:

• the cum clause comes first to set the scene
• sonitus appears early in the main clause as the subject
• mutatur comes at the end, a very common and natural place for a Latin verb

So the order is not strange in Latin, even if it feels different from English.