La diagnosi della dottoressa mi tranquillizza, perché non è nulla di grave.

Yes. Diagnosi is a feminine noun: la diagnosi.

This can look strange at first, because many Italian nouns ending in -i are plural. But some singular nouns of Greek origin end in -si and are feminine, such as:

• la diagnosi
• l’analisi
• la crisi

So in this sentence, la diagnosi is singular and feminine.

Della is di + la, meaning of the.

So:

• la diagnosi della dottoressa = the doctor's diagnosis
• literally: the diagnosis of the doctor

Italian often uses di + noun where English uses 's.

Examples:

• il libro di Marco = Marco’s book
• la macchina della professoressa = the teacher’s car

So this structure is very normal.

In Italian, when you mean of the doctor, you normally use the article: della dottoressa.

Using di dottoressa would usually sound incomplete or unnatural here. The article helps specify the person.

So:

• della dottoressa = of the doctor / the doctor’s
• not normally di dottoressa in this sentence

This is one of the places where Italian uses articles more often than English.

Mi means me.

So mi tranquillizza means:

• reassures me
• calms me down
• literally, makes me calm

The pronoun comes before the conjugated verb in Italian:

• mi tranquillizza = it reassures me
• ti tranquillizza = it reassures you
• ci tranquillizza = it reassures us

Not usually. This is a helpful false-friend warning.

In this sentence, tranquillizza means:

• reassures
• puts at ease
• calms

It does not normally mean medically sedating someone.

So La diagnosi della dottoressa mi tranquillizza means the diagnosis makes the speaker feel calmer or less worried.

Because the subject is la diagnosi, not the speaker.

The sentence says that the diagnosis reassures me:

• La diagnosi ... mi tranquillizza
  = The diagnosis reassures me

If you said mi tranquillizzo, that would mean:

• I calm down
• I become calmer

So the difference is:

• mi tranquillizza = something calms/reassures me
• mi tranquillizzo = I calm down

The comma separates the main idea from the explanation.

• Main clause: La diagnosi della dottoressa mi tranquillizza
• Reason clause: perché non è nulla di grave

So the comma works like:

• The doctor’s diagnosis reassures me, because it’s nothing serious

In Italian, this comma is very natural and standard in a sentence like this.

Because standard Italian spells perché with an accent on the final é.

That is true whether it means:

• because
• why

Examples:

• Sono contento perché sei qui. = I’m happy because you’re here.
• Perché sei qui? = Why are you here?

So in your sentence, perché means because, and the accent is required.

Literally, it means something like:

• it is nothing serious
• more literally: it is nothing of a serious kind

Here:

• non è = it is not
• nulla = nothing
• di grave = serious

As a whole, it means that the problem or condition is not serious.

After words like nulla, niente, qualcosa, and molto, Italian often uses di + adjective.

So:

• nulla di grave = nothing serious
• qualcosa di interessante = something interesting
• niente di nuovo = nothing new

This is a very common pattern, and it often sounds more natural than trying to put the adjective directly after the pronoun.

Yes. In everyday Italian, nulla and niente are often interchangeable here.

So you could also say:

• non è niente di grave

That would still mean:

• it’s nothing serious

Very roughly:

• niente is often more common in spoken Italian
• nulla can sound a bit more neutral or slightly more formal in some contexts

But both are perfectly normal.

It refers to the thing that was being diagnosed: the illness, symptom, problem, or condition.

Italian does not always need to state that explicitly. English does the same with it:

• It’s nothing serious

So even though the sentence does not name the problem again, it is understood from context.

Yes. That is also correct.

Both are grammatical:

• La diagnosi della dottoressa mi tranquillizza
• Mi tranquillizza la diagnosi della dottoressa

The difference is mainly emphasis:

• La diagnosi della dottoressa... starts by highlighting the doctor’s diagnosis
• Mi tranquillizza... starts by highlighting the effect on me

Italian word order is often more flexible than English, especially for emphasis or style.