La supplente detta tre domande, e noi le scriviamo sul quaderno.

Supplente means substitute teacher or supply teacher. In this sentence, la shows that the teacher is female, so la supplente means the female substitute teacher.

A few useful points:

• supplente can be masculine or feminine depending on the article:
  • il supplente = the male substitute teacher
  • la supplente = the female substitute teacher
• The noun itself does not change here; the article tells you the gender.

Because dettare and dire are not the same thing.

• dire = to say, to tell
• dettare = to dictate

So La supplente detta tre domande means that the teacher is dictating three questions for the students to write down, not just saying them in a general way.

Here, detta is the third-person singular present of dettare:

• io detto
• tu detti
• lui/lei detta

In Italian, after a number, you normally do not use an article.

So:

• tre domande = three questions
• not le tre domande, unless you specifically mean the three questions

Compare:

• detta tre domande = she dictates three questions
• detta le tre domande = she dictates the three questions

The version in your sentence is just the normal, neutral way to say it.

Le is a direct object pronoun meaning them, and it refers back to tre domande.

Why le?

• domanda is feminine singular
• domande is feminine plural
• the direct object pronoun for feminine plural is le

So:

• tre domande → le
• noi le scriviamo = we write them

Because in Italian, object pronouns usually come before a conjugated verb.

So Italian says:

• Noi le scriviamo

Literally, that is something like:

• We them write

But in natural English, of course, you say:

• We write them

This word order is very normal in Italian:

• Lo vedo = I see him/it
• La compro = I buy it
• Le scriviamo = we write them

Yes, it could. Italian often leaves out subject pronouns because the verb ending already shows who the subject is.

• scriviamo already means we write
• so le scriviamo is enough

If noi is included, it often adds:

• emphasis
• clarity
• contrast

So e noi le scriviamo can sound a bit like:

• and we write them down
• and we are the ones who write them

Sul quaderno means on the notebook or, more naturally in English, in the notebook.

It is made from:

• su = on
• il = the
• su + il = sul

So:

• sul quaderno = on the notebook / in the notebook

In this context, English usually says in the notebook, but Italian commonly uses su with things you write on, such as pages, paper, or a notebook.

Because with writing, Italian often prefers su when talking about the surface or pages where something is written.

So:

• scrivere sul quaderno is very natural

Nel quaderno is not impossible, but sul quaderno is the more typical choice here when talking about writing something down in your notebook.

You will often see the same pattern with other words:

• scrivere sul foglio = write on the sheet of paper
• scrivere sulla lavagna = write on the board

Scriviamo is the first-person plural present tense of scrivere (to write).

The present tense forms are:

• io scrivo = I write
• tu scrivi = you write
• lui/lei scrive = he/she writes
• noi scriviamo = we write
• voi scrivete = you all write
• loro scrivono = they write

So noi scriviamo means we write.

Yes. Both verbs are in the present tense:

• detta = dictates
• scriviamo = we write

In Italian, the present tense can describe:

• something happening right now
• a habitual or repeated action
• a general classroom routine

So this sentence could mean:

• the substitute teacher is dictating three questions right now, and we are writing them down or
• the substitute teacher dictates three questions, and we write them down

The exact nuance depends on context.

Usually quaderno means a school notebook or exercise book, especially the kind students write in during class.

It is a very common school word. In many contexts, translating it as notebook is perfectly fine, but in a classroom sentence like this, exercise book is also a good fit.

So sul quaderno suggests writing the dictated questions in your school notebook.