Magistra dicit terminationem huius vocabuli in plurali mutari.

Because this sentence uses an indirect statement after dicit (says).

In Latin, after verbs of saying, thinking, knowing, and similar verbs, English often uses that + a finite verb:

• The teacher says that the ending changes.

But Latin usually uses:

• accusative + infinitive

So in terminationem ... mutari:

• terminationem = the subject of the indirect statement, but it appears in the accusative
• mutari = the infinitive, to be changed / to change

So although terminationem is logically the thing that changes, grammatically it is in the accusative because of this construction.

For the same reason: Latin indirect statement.

After dicit, Latin does not normally say:

• Magistra dicit terminatio mutatur

Instead it says:

• Magistra dicit terminationem mutari

Literally, this is something like:

• The teacher says the ending to be changed

Natural English turns that into:

• The teacher says that the ending changes / is changed

So mutari is the present passive infinitive of muto.

Mutari is present passive infinitive.

From muto, mutare = to change

Its passive infinitive is:

• mutari = to be changed

So the phrase means:

• terminationem ... mutari = that the ending is changed / changes

In English, depending on context, we may translate more naturally as changes, even though the Latin form is passive.

It means of this word.

• vocabuli is the genitive singular of vocabulum
• huius is also genitive singular, agreeing with vocabuli

So:

• terminatio huius vocabuli = the ending of this word

This is a very normal use of the genitive in Latin: showing possession or close relationship.

Because the sentence needs of this word, not this word.

Compare:

• hoc vocabulum = this word
  
  • nominative or accusative neuter singular
• huius vocabuli = of this word
  
  • genitive singular

Since terminationem is the ending, Latin needs a phrase meaning of this word to explain which ending. That requires the genitive:

• terminationem huius vocabuli = the ending of this word

It means in the plural.

This is an idiomatic grammatical expression. The adjective pluralis is being used almost like a noun, with something understood, such as numero (number):

• in plurali = in the plural (number)

So the idea is:

• the ending of this word changes in the plural

The preposition in takes the ablative here because it means in in a non-motion sense.

Because it follows in, and here in means in or within, not motion into something.

Latin usually works like this:

• in
  • accusative = into, onto
• in
  • ablative = in, on

So:

• in plurali = in the plural

This is an ablative phrase of circumstance or grammatical category.

Yes.

• Magistra is nominative singular
• dicit is third-person singular present

So:

• Magistra dicit = The teacher says

Since magistra is feminine, it specifically means a female teacher or mistress/teacher, depending on context, but in a classroom sentence teacher is the natural translation.

Because Latin usually does not need subject pronouns unless they are being emphasized.

The ending of dicit already tells you it is:

• third person singular = he/she/it says

Since magistra is present, the subject is already completely clear:

• Magistra dicit = The teacher says

Adding a pronoun would usually be unnecessary unless you wanted emphasis or contrast.

It breaks down like this:

• Magistra = subject of the main verb
• dicit = main verb
• terminationem huius vocabuli in plurali mutari = indirect statement

Inside the indirect statement:

• terminationem = accusative subject of the infinitive
• huius vocabuli = genitive phrase, of this word
• in plurali = in the plural
• mutari = present passive infinitive

So the structure is:

• The teacher says [the ending of this word to be changed in the plural].

Natural English:

• The teacher says that the ending of this word changes in the plural.

It may feel unusual to an English speaker, but it is normal Latin.

Latin word order is much freer than English word order because the case endings show how the words relate to each other.

This sentence places the infinitive mutari at the end, which is very common in Latin, especially in infinitive clauses.

Also, Latin often places related words near each other for emphasis or clarity:

• terminationem huius vocabuli = the ending of this word
• in plurali = in the plural
• mutari at the end rounds off the indirect statement

So the order is perfectly natural Latin, even if English would structure it differently.

Yes.

Dicit is present tense, and Latin present tense can often be translated in more than one way depending on context:

• says
• is saying

In a general statement like this, says is the most natural translation:

• The teacher says that the ending of this word changes in the plural.

Because that is one of Latin’s standard ways of reporting statements.

English commonly says:

• She says that the ending changes.

Latin very often prefers:

• She says the ending to change / to be changed

That is, Latin uses:

• a verb of saying, thinking, knowing, etc.
• then an accusative subject
• then an infinitive

This construction is one of the most important features of Latin syntax, and this sentence is a good example of it.