Magistra dicit hanc sententiam alio modo explicandam esse.

Because this sentence uses an indirect statement after dicit.

In Latin, after verbs like dicit (says), the content of what is said is often expressed with:

• an accusative subject
• plus an infinitive

So hanc sententiam is not the direct object of dicit in the ordinary English sense. It is the subject of the infinitive phrase explicandam esse.

Grammatically:

• hanc = accusative feminine singular
• sententiam = accusative feminine singular

They match because hanc modifies sententiam.

It is the Latin accusative-and-infinitive construction, often called an indirect statement.

So instead of saying something like:

• The teacher says that this sentence must be explained...

Latin says more literally:

• The teacher says this sentence to be needing to be explained...

That sounds awkward in English, but it is normal Latin structure.

Here:

• dicit = the main verb, says
• hanc sententiam = the accusative subject of the reported statement
• explicandam esse = the infinitive part of the reported statement

Because explicandam esse means something very different from explicare.

• explicare = to explain
• explicandam esse = to have to be explained, must be explained, is to be explained

This is a gerundive + esse construction, often called the passive periphrastic. It expresses necessity or obligation.

So the sentence is not saying:

• The teacher says this sentence to explain...

It is saying:

• The teacher says that this sentence must be explained...

Explicandam is a gerundive of explico, explicare.

A gerundive is a verbal adjective. Here it agrees with sententiam, so it is:

• feminine
• singular
• accusative

That is why it is explicandam and not some other form.

Agreement:

• hanc sententiam = feminine singular accusative
• explicandam = feminine singular accusative

So explicandam modifies sententiam and means something like needing to be explained.

Esse is needed because the construction is gerundive + esse.

The gerundive by itself gives the idea of needing to be explained, but the full passive periphrastic uses a form of sum:

• explicanda est = must be explained
• explicandam esse = to have to be explained

Since this whole clause depends on dicit, Latin uses the infinitive esse, not a finite verb like est.

So:

• explicandam est would not fit here
• explicandam esse is correct in indirect statement

Alio modo is an ablative phrase meaning in another way or by another method.

Breakdown:

• alio = ablative singular of alius, other / another
• modo = ablative singular of modus, way / manner / method

Together they form an idiomatic expression:

• alio modo = in another way

A natural English translation might also be:

• differently

No. Magistra is definitely the one who says it, because she is the subject of dicit. But she is not necessarily the one who must do the explaining.

The sentence only says that the sentence must be explained in another way. It does not state who has the obligation.

If Latin wanted to name the person who had the duty, it could use a dative of agent with the passive periphrastic.

For example, something like:

• nobis explicanda est = it must be explained by us / we must explain it

But in your sentence, no such agent is stated.

Latin word order is much more flexible than English word order because endings show grammatical relationships.

English relies heavily on position:

• The teacher says that...

Latin can move words around more freely for emphasis or style.

In this sentence, the order is quite natural:

• Magistra first: the speaker is introduced
• dicit next: the main action
• hanc sententiam: what the reported statement is about
• alio modo: the manner
• explicandam esse at the end: the key verbal idea of the indirect statement

You could rearrange parts of the sentence and still keep the same basic meaning, though the emphasis might change.

Because hanc is the accusative feminine singular form, while haec is usually nominative feminine singular.

Since sententiam is accusative feminine singular, the demonstrative must match it:

• nominative: haec sententia
• accusative: hanc sententiam

Because the noun is accusative in the indirect statement, the demonstrative must also be accusative.

Originally, sententia often means thought, opinion, view, or judgment. But in later and classroom Latin, it is also commonly used for a sentence in the grammatical sense.

So a learner may notice that the word has a broader meaning than English sentence. Context tells you which sense is intended.

Here, since the meaning has already been given to the learner, sententiam is clearly being used in the sense of a written or spoken sentence that needs explanation.

A useful literal breakdown is:

• Magistra = the teacher
• dicit = says
• hanc sententiam = this sentence
• alio modo = in another way
• explicandam esse = to have to be explained

So, very literally:

• The teacher says this sentence to have to be explained in another way.

That is not natural English, but it helps show the Latin grammar.

A smoother English version would be:

• The teacher says that this sentence must be explained in another way.