Magistra dicit imperfectum ad tempus praeteritum pertinere.

Because Latin often expresses that-clauses after verbs of saying, thinking, knowing, and perceiving with an indirect statement construction instead of a conjunction.

Here, the indirect statement is:

imperfectum ad tempus praeteritum pertinere

So Latin says, literally, something like:

The teacher says the imperfect to pertain to past time

where English would say:

The teacher says that the imperfect pertains to past time.

This is one of the most important Latin patterns to learn:

• verb of saying/thinking/perceiving
• accusative subject
• infinitive verb

Because it is the verb inside the indirect statement.

After dicit, Latin normally uses:

• a subject in the accusative
• a verb in the infinitive

So:

• dicit = the main verb, says
• pertinere = the verb of what is being said, to pertain / to belong

If Latin used pertinet, that would usually make it a separate direct statement:

• Magistra dicit: imperfectum ad tempus praeteritum pertinet.

But in your sentence, Latin has folded that idea into indirect statement form.

It is accusative singular, because it is the subject of the infinitive pertinere in an indirect statement.

That may feel strange at first, because in English the subject would still look like a subject:

• the imperfect belongs...

But in Latin indirect statement, the logical subject goes into the accusative.

A very important extra point: imperfectum looks the same in the nominative and accusative, because it is neuter singular. So the form itself does not visibly change.

Because neuter singular forms often have identical nominative and accusative endings.

So:

• nominative neuter singular: imperfectum
• accusative neuter singular: imperfectum

That means you have to identify its role from the construction, not just from the form by itself.

Here the construction after dicit tells you it is accusative:

• dicit
  • accusative + infinitive

Here it means the grammatical tense, the imperfect.

Latin often uses adjectives as nouns when the context makes the meaning clear. In grammar, imperfectum can stand for something like:

• tempus imperfectum = the imperfect tense

So the sentence is talking about grammar terminology, not about something being incomplete in a general sense.

It is a prepositional phrase explaining what the imperfect pertains to.

Break it down:

• ad = to, toward, in relation to
• tempus = time
• praeteritum = past

With pertinere, ad often means to pertain to, to relate to, to belong to in the sense of category or reference.

So:

• ad tempus praeteritum pertinere = to pertain to past time

Because it follows the preposition ad, which takes the accusative.

So:

• ad
  • accusative

That gives:

• ad tempus

And praeteritum matches tempus in case, number, and gender:

• tempus = neuter singular accusative
• praeteritum = neuter singular accusative

Because it agrees with tempus.

Latin adjectives must agree with the nouns they modify in:

• gender
• number
• case

Here:

• tempus is neuter singular accusative
• therefore praeteritum is also neuter singular accusative

So tempus praeteritum means past time.

dicit is present indicative active, third person singular:

• she says
• or the teacher says

Yes, it affects how we understand pertinere.

In indirect statement, the tense of the infinitive is usually relative to the main verb:

• present infinitive = same time as the main verb
• perfect infinitive = earlier than the main verb
• future infinitive = later than the main verb

So pertinere as a present infinitive means the belonging/pertaining is understood as simultaneous with dicit:

• she says that it pertains

The fact that the phrase contains past time does not make the infinitive itself past. Past is just part of the idea being described.

Because Classical Latin has no articles.

So Latin does not have separate words for:

• the
• a
• an

That means a form like magistra can mean:

• teacher
• the teacher
• a teacher

and imperfectum can mean:

• the imperfect
• sometimes just imperfect, depending on context

English has to add articles when translating, but Latin does not write them.

Latin word order is more flexible than English word order because Latin uses endings to mark grammatical relationships.

This sentence is arranged in a very readable way:

• Magistra = subject
• dicit = main verb
• then the whole indirect statement: imperfectum ad tempus praeteritum pertinere

A different order could still be grammatical, but the emphasis might change. For example, Latin could move words around for style or focus. What matters most is:

• magistra is nominative
• dicit is the main finite verb
• imperfectum is the accusative subject of the infinitive
• pertinere is the infinitive

So the endings and construction tell you the grammar more than the word order does.

Yes. pertinere ad is a standard expression meaning things like:

• to pertain to
• to relate to
• to belong to
• to concern

So in this sentence, the idea is that the imperfect tense belongs in the category of past time, or is related to past time.

This is a useful phrase to recognize as a unit:

• pertinet ad
• pertinere ad

rather than translating ad too mechanically as only to or toward.

It breaks down like this:

• Magistra — nominative singular, the subject of the main verb
• dicit — present indicative active, main verb
• imperfectum — accusative singular, subject of the infinitive in indirect statement
• ad tempus praeteritum — prepositional phrase modifying pertinere
• pertinere — present active infinitive, verb of the indirect statement

So the pattern is:

Main clause
Magistra dicit

plus

Indirect statement
imperfectum ad tempus praeteritum pertinere

That is a very common and very important Latin sentence pattern.