In atrio puella tibiam sonare audit et ad ostium currit.

Because in changes meaning depending on the case that follows it:

• in + ablative = location, in / inside
• in + accusative = motion into, into

Here the girl is already in the atrium, so Latin uses the ablative: in atrio.

After verbs of perception such as audio (hear), Latin often uses an infinitive to describe the action being perceived. English can do something similar:

• I hear the bell ring
• She hears the flute sound

So sonare depends on audit; it is not the main verb of the sentence.

Because it belongs to the construction with audit and sonare. After verbs like audio, Latin often uses an accusative-plus-infinitive pattern. Here tibiam is the accusative noun connected to sonare.

A very literal way to understand it is:

• puella tibiam sonare audit = the girl hears the flute sound

So tibiam is part of what she hears happening.

Not in the simplest grammatical analysis. It is usually taken as the accusative subject of the infinitive sonare in the perception construction after audit.

That can feel strange to an English speaker, but English has something a bit similar in sentences like:

• I saw him run
• We heard the bell ring

Latin uses that kind of structure very often.

Ad takes the accusative and shows movement toward something. So ad ostium means to the door or toward the doorway.

Also, ostium is a neuter second-declension noun, so its nominative singular and accusative singular look the same. Here it is accusative because of ad.

They express two different ideas:

• in atrio = where she is
• ad ostium = where she goes

Latin is very precise about location versus motion:

• in + ablative for being in a place
• ad + accusative for movement toward a place

Latin word order is much freer than English word order because Latin endings show how words function in the sentence. English depends heavily on word order; Latin depends more on case endings and verb endings.

So Latin can begin with in atrio to set the scene, then give the subject puella, then the action she hears, and then what she does next.

Both are present indicative, third-person singular:

• audit = he/she hears
• currit = he/she runs

So the sentence is describing actions in the present: she hears and she runs.