Nondum plane intellego cur hoc verbum in coniunctivo positum sit.

Because cur hoc verbum in coniunctivo positum sit is an indirect question depending on intellego.

A very common Latin rule is:

• direct question → often indicative
• indirect question → normally subjunctive

So compare:

• Cur hoc verbum in coniunctivo positum est? = Why has this word been put in the subjunctive?
• Nondum plane intellego cur hoc verbum in coniunctivo positum sit. = I do not yet fully understand why this word has been put in the subjunctive.

Once the question is reported indirectly after intellego, Latin uses the subjunctive.

Positum sit is the perfect passive subjunctive, third person singular.

It is made from:

• positum = perfect passive participle of pono
• sit = present subjunctive of sum

Together they mean has been placed / has been put in the subjunctive.

So the structure is:

• hoc verbum ... positum sit = this word has been placed ...

Because sit by itself only means is / may be.
The idea here is not simply why this word is in the subjunctive, but more specifically why this word has been put/placed in the subjunctive.

Latin often uses pono in grammatical discussion for placing a word in a certain case, mood, tense, position, and so on.

So:

• in coniunctivo positum sit = has been put in the subjunctive

This is a standard and natural way to talk about grammatical form.

Because the word has already been written or used in that form. The placing is viewed as prior to the speaker’s present attempt to understand it.

The main verb is present:

• intellego = I understand

The subordinate verb refers to an earlier completed action:

• positum sit = has been placed / was placed

So the sense is: I do not yet fully understand why this word has been put in the subjunctive.

If Latin used a present subjunctive form such as ponatur, that would sound more like why this word is being put or why it is put in a more general or less completed sense.

Cur means why and introduces the question clause.

In this sentence, it introduces an indirect question:

• cur hoc verbum in coniunctivo positum sit
  = why this word has been put in the subjunctive

So cur is the question word, and the whole clause after it functions as the object of intellego: what the speaker does not yet fully understand is why this happened.

It means in the subjunctive.

Here:

• in takes the ablative
• coniunctivo is ablative singular

This is an idiomatic way to talk about grammatical category or form:

• in coniunctivo = in the subjunctive
• compare similar English phrasing: in the accusative, in the passive, in the plural

The noun modo is not expressed, but the idea is essentially in the subjunctive mood.

Because the normal Latin grammatical term is coniunctivus.

English subjunctive comes through a different historical path, but Latin grammarians commonly use:

• coniunctivus = subjunctive

So:

• in coniunctivo = in the subjunctive

This is simply the standard Latin term.

Positum agrees with hoc verbum.

Both are:

• neuter
• singular
• nominative

That is because hoc verbum is the subject of the passive verb:

• hoc verbum ... positum sit = this word has been placed ...

Since verbum is neuter singular, the participle must also be neuter singular: positum.

Because verbum is a neuter noun of the second declension.

Its nominative singular is:

• verbum = word

The demonstrative hic, haec, hoc must agree with it, so the correct form is:

• hoc verbum = this word

Not hic verbum or haec verbum, because those would not match the gender.

Both words modify intellego.

• nondum = not yet
• plane = fully, clearly, completely

So:

• Nondum plane intellego = I do not yet fully understand

The combination suggests that the speaker may understand the issue partly, but not completely.

Because Latin plane often means plainly, clearly, completely, quite depending on context.

With a verb like intellego, it naturally means something like:

• clearly
• fully
• completely

So nondum plane intellego is a very natural Latin way to say I do not yet fully understand.

Not really. It is very natural Latin word order.

Latin often puts adverbs early:

• Nondum plane intellego = I do not yet fully understand

Then it places the indirect question afterward:

• cur hoc verbum in coniunctivo positum sit

Latin word order is flexible, but this arrangement is clear and idiomatic:

1. main statement first
2. dependent question after it

So the sentence is both normal and elegant.

In standard Latin, an indirect question like this normally takes the subjunctive, not the indicative.

So after intellego, the expected construction is:

• cur ... sit

not

• cur ... est

The indicative would belong more naturally in a direct question:

• Cur hoc verbum in coniunctivo positum est?

But once that question becomes dependent on intellego, scio, nescio, quaero, and similar verbs, Latin normally changes to the subjunctive.

Yes, but it would not mean exactly the same thing.

• cur hoc verbum in coniunctivo positum sit = why this word has been put in the subjunctive
• cur hoc verbum in coniunctivo ponatur = why this word is put / is being put in the subjunctive

The first points to the word as already appearing in that form; the second sounds more general or more in process.

In grammatical explanation, positum sit is very natural when discussing a word already found in a text.