Magistra ostendit hoc verbum in indicativo positum esse, illud autem in coniunctivo.

The sentence has two layers:

• the main clause: Magistra ostendit
• the thing being shown: hoc verbum in indicativo positum esse, illud autem in coniunctivo

So the teacher is showing that...

This second part is an indirect statement, a very common Latin construction. Instead of using that plus a finite verb, Latin usually uses:

• an accusative subject
• an infinitive verb

Here:

• hoc verbum = the subject of the first part of the indirect statement
• positum esse = the infinitive
• illud = the subject of the second, understood parallel part

So the sentence means, structurally:

• The teacher shows [this word to have been placed in the indicative, but that one in the subjunctive].

Because after ostendit Latin often uses indirect statement.

In English we say:

• The teacher shows that this word is in the indicative.

Latin does not normally say quod hoc verbum... est here. Instead it prefers:

• hoc verbum ... positum esse

This is the standard accusative + infinitive construction.

Compare:

• direct statement: Hoc verbum in indicativo positum est.
• indirect statement after ostendit: Magistra ostendit hoc verbum in indicativo positum esse.

So esse is there because the whole clause has been turned into an infinitive statement.

Because it agrees with verbum.

• verbum is a neuter singular noun
• therefore its participle must also be neuter singular
• so we get positum

If the noun were feminine singular, you would expect positam; if masculine singular, positum would change accordingly.

Here:

• hoc verbum ... positum esse = this word to have been placed / to be set

The participle is from pono, ponere, posui, positum.

Because they refer to verbum, which is neuter.

• hoc = this
• illud = that

Both are neuter singular accusative here, because they are standing for:

• hoc verbum
• illud verbum

Latin often omits a noun when it is easy to understand from context. So the second half does not need to repeat verbum.

They are both ablative singular.

That is because they follow in:

• in indicativo
• in coniunctivo

Here in means in in the sense of in the indicative mood / in the subjunctive mood, and it takes the ablative.

So:

• indicativus, -i -> indicativo
• coniunctivus, -i -> coniunctivo

This is a very standard way to describe grammatical form in Latin.

Yes. Literally, positum means placed or put.

So:

• in indicativo positum esse literally means to have been placed in the indicative
• idiomatically, it means to be in the indicative or to be put in the indicative form

Latin often uses concrete verbs like pono in grammatical or rhetorical descriptions.

So even though the literal image is placed, the natural sense is simply used in or found in that mood.

Because Latin often leaves out words that are easily understood from the first part. This is called ellipsis.

The full version would be something like:

• Magistra ostendit hoc verbum in indicativo positum esse, illud autem in coniunctivo positum esse.

But once positum esse has already appeared, Latin can omit it in the second half.

Likewise, verbum is also understood after illud.

So the second half really means:

• illud verbum autem in coniunctivo positum esse

Latin likes this kind of compression when the pattern is clear.

Autem means something like:

• however
• but
• on the other hand

Here it marks a contrast between the two words:

• this word is in the indicative
• that one, however, is in the subjunctive

A useful thing to remember is that autem is usually postpositive: it tends to come second in its clause or phrase, not first.

So Latin says:

• illud autem not
• autem illud

Formally, ostendit can be either:

• present: he/she shows
• perfect: he/she showed / has shown

This is a common feature of the third-conjugation perfect in Latin: the perfect active third singular can look just like the present.

So:

• magistra ostendit could mean the teacher shows
• or the teacher showed

Context normally tells you which one is intended.

In a grammatical explanation like this, the present sense shows is often the most natural, but either is possible depending on context.

Because magistra is the subject of ostendit.

• magistra = the female teacher
• nominative singular feminine

The ending -a here is the normal nominative singular ending of a first-declension noun.

So:

• Magistra ostendit = The teacher shows

A direct version would use finite verbs:

• Hoc verbum in indicativo positum est, illud autem in coniunctivo positum est.

Or with the second positum est omitted:

• Hoc verbum in indicativo positum est, illud autem in coniunctivo.

Then, when Latin turns this into an indirect statement after ostendit, positum est becomes positum esse, and the subjects become accusative:

• hoc verbum ... positum esse
• illud ... [positum esse]

This is a very useful transformation to recognize when reading Latin.

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

English depends heavily on position:

• The teacher shows that this word is in the indicative...

Latin depends much more on endings:

• Magistra tells you the subject
• hoc verbum tells you the accusative subject of the indirect statement
• indicativo / coniunctivo show their case through their endings
• positum esse marks the infinitive construction

That means Latin can arrange words for emphasis or balance.

This sentence is arranged very neatly:

• hoc verbum in indicativo
• illud autem in coniunctivo

That parallel structure makes the contrast clear and elegant.