Se l’insegnante detta troppo in fretta, non riesco a sottolineare tutto.

Because insegnante begins with a vowel, Italian normally uses the shortened article l’ before it.

So you get:

• l’insegnante = the teacher

If you need to make the gender clear elsewhere, context usually does it. For example:

• l’insegnante è bravo = the teacher is male
• l’insegnante è brava = the teacher is female

The full forms il insegnante and la insegnante are not used here.

It can be either masculine or feminine, depending on the teacher.

Insegnante is a noun that has the same singular form for both genders:

• l’insegnante = the male teacher / the female teacher

You usually understand the gender from:

• context
• adjectives
• past participles
• pronouns

For example:

• L’insegnante è stanco. = The teacher is tired. (male)
• L’insegnante è stanca. = The teacher is tired. (female)

No—dettare means to dictate, not just to say.

So:

• l’insegnante detta = the teacher dictates

This is the verb you use when someone speaks so that others can write the words down.

Compare:

• dire = to say
• parlare = to speak
• dettare = to dictate

So this sentence is specifically about a teacher speaking for students to write, not just talking in general.

Yes, both are possible, but troppo in fretta is very natural and common.

• in fretta = quickly / in a hurry
• troppo in fretta = too quickly / too fast

You could also say:

• troppo velocemente

That is correct too, but in fretta often sounds more everyday and idiomatic in speech.

This is a very common real-condition structure in Italian:

• Se + present tense, present tense

So:

• Se l’insegnante detta troppo in fretta, non riesco a sottolineare tutto.

This means something like:

• If the teacher dictates too quickly, I can’t underline everything.

It describes a real or likely situation, not a hypothetical one.

Other examples:

• Se piove, resto a casa. = If it rains, I stay home.
• Se studi, impari. = If you study, you learn.

Because Italian usually does not need subject pronouns when the verb ending already shows who the subject is.

• riesco = I manage / I can
• The ending -o tells you it is io

So:

• non riesco = I can’t manage / I’m not able

Adding io is possible, but usually only for emphasis or contrast:

• Io non riesco, ma lui sì. = I can’t, but he can.

Riuscire a + infinitive means to manage to, to succeed in, or to be able to in the sense of actually managing something.

So:

• non riesco a sottolineare tutto = I can’t manage to underline everything

This is slightly different from non posso, which usually means:

• I am not allowed to
• it is not possible for some external reason

Compare:

• Non posso parlare. = I can’t speak / I’m not allowed to speak / I’m unable to speak
• Non riesco a parlare. = I can’t manage to speak

In your sentence, non riesco is a good choice because the problem is that the teacher is going too fast, so the speaker cannot keep up.

Because riuscire normally takes a + infinitive.

So the pattern is:

• riuscire a fare qualcosa = to manage to do something

Examples:

• Riesco a capire. = I manage to understand.
• Non riesce a dormire. = He/She can’t manage to sleep.
• Riusciamo a finire il lavoro. = We manage to finish the work.

So a is required after riuscire in this structure.

Here it means to underline, as in drawing a line under words in a text.

• sottolineare tutto = to underline everything

But sottolineare can also mean to emphasize in more abstract contexts.

For example:

• Vorrei sottolineare un punto importante. = I’d like to emphasize an important point.

So it has both a literal meaning and a figurative one. In your sentence, the literal meaning is intended.

Here tutto means everything.

When tutto is used as a pronoun meaning everything, it is normally masculine singular:

• tutto = everything

So:

• sottolineare tutto = to underline everything

It does not have to agree with insegnante. It refers to the whole amount of material being dictated, not to the teacher.

Compare:

• Ho letto tutto. = I read everything.
• Ho capito tutto. = I understood everything.

You would normally use parlare:

• Se l’insegnante parla troppo in fretta, non riesco a sottolineare tutto.

That means:

• If the teacher speaks too fast, I can’t manage to underline everything.

The original sentence with detta is more specific: it suggests the teacher is giving words for students to write down.

The apostrophe in l’insegnante shows elision: the final vowel of the article drops before the next vowel.

So instead of saying something like la insegnante, Italian contracts it to:

• l’insegnante

The l’ connects directly to the next word.

A rough pronunciation guide:

• l’insegnante ≈ leen-se-nyan-te
• detta ≈ DET-ta
• troppo in fretta ≈ TROP-po in FRET-ta

A learner should especially notice:

• gli / gn-type sounds in Italian can be tricky, and insegnante contains gn, pronounced like ny in canyon
• detta has a doubled tt, so the t is held a little longer than in English

Yes, it is grammatical, but it changes the nuance.

• non riesco a sottolineare tutto = I can’t manage to underline everything
• non posso sottolineare tutto = I can’t underline everything

The second one is broader and can sound like:

• I’m not able to
• I’m not allowed to
• it isn’t possible

The original sentence with non riesco a is more precise because it suggests difficulty in keeping up with the teacher’s speed.