Marcus rogat cur in sententia passiva obiectum saepe in subiectum vertatur.

Because cur ... vertatur is an indirect question after rogat.

Latin regularly uses the subjunctive in indirect questions:

• Marcus rogat ... = Marcus asks ...
• cur ... vertatur = why ... is turned / gets turned

So the subjunctive here is not mainly about doubt or unreality; it is required by the construction.

It is also present subjunctive because the main verb rogat is present, and the turning is happening at the same time as the asking. If the main verb were past, you would often expect verteretur instead.

The sentence has two parts:

• Main clause: Marcus rogat
• Indirect question: cur in sententia passiva obiectum saepe in subiectum vertatur

So literally the structure is:

• Marcus asks
• why, in a passive sentence, the object is often turned into the subject

This is a very common Latin pattern: a main verb of asking, knowing, seeing, hearing, etc., followed by an indirect question.

Because obiectum is the subject of the verb vertatur in this sentence.

That can feel strange at first, but the word object here is being used as a grammatical label, not as the grammatical function of the word inside this Latin sentence.

So:

• obiectum = the thing being discussed
• grammatically, in this sentence, obiectum is nominative because it is what is being turned

A helpful English comparison is:

• The object becomes the subject.

In that English sentence, the object is also the subject of becomes.

Because in takes different cases depending on the meaning:

• in + ablative = in, within, in the context of
• in + accusative = into, onto, toward

So here:

• in sententia passiva = in a passive sentence
  This is a static setting, so Latin uses the ablative.

• in subiectum = into a subject
  This shows change toward a new state, so Latin uses the accusative.

This is a very important Latin pattern.

Vertatur is:

• 3rd person singular
• present
• subjunctive
• passive

from vertere = to turn, change, transform

So vertatur means:

• is turned
• may be turned
• in this context, more naturally, is turned or gets turned

The subject is obiectum, so:

• obiectum ... vertatur = the object is turned

Because passiva is an adjective agreeing with sententia.

In Latin, adjectives agree with the nouns they describe in:

• gender
• number
• case

Here:

• sententia is feminine singular ablative
• so passiva must also be feminine singular ablative

That is why you get sententia passiva, not sententia passivum or some other form.

Saepe is an adverb meaning often.

It modifies the verb vertatur:

• obiectum saepe in subiectum vertatur
• the object is often turned into the subject

Its position is flexible. Latin word order is freer than English word order, so saepe can move around without changing the basic meaning, though some positions may sound more natural or emphasize different parts.

No. Latin word order is relatively flexible because the endings show the grammatical relationships.

This sentence could be rearranged in various ways and still mean the same basic thing, for example by moving saepe or the prepositional phrases. The given order is simply a clear and natural one.

A learner should focus first on:

• verb forms
• case endings
• clause structure

rather than assuming Latin word order works like English word order.

Yes, in grammatical or pedagogical Latin it can.

A native English speaker may have learned that sententia often means things like:

• opinion
• view
• thought
• maxim

That is true, especially in more classical usage. But in later, scholarly, and teaching Latin, sententia can also be used for a sentence in the grammatical sense.

Similarly, obiectum and subiectum here are grammatical terms. They are perfectly understandable in this context, even if they feel a bit technical or post-classical.

Because rogat is the main verb of a straightforward statement:

• Marcus rogat = Marcus asks

There is no reason for a subjunctive in the main clause here. The subjunctive appears only in the subordinate indirect question:

• cur ... vertatur

So the pattern is:

• main statement: indicative
• indirect question inside it: subjunctive