Mi sono innamorato di lei.

Because the verb is innamorarsi, not innamorare.

• innamorarsi di qualcuno = to fall in love with someone
• It is a pronominal/reflexive-type verb, so in the passato prossimo it uses essere, not avere.

So:

• mi sono innamorato = I fell in love
• not ho innamorato, which would not mean the same thing

A useful pattern is:

• mi sono innamorato/a
• ti sei innamorato/a
• si è innamorato/a
• ci siamo innamorati/e
• vi siete innamorati/e
• si sono innamorati/e

Mi is the first-person singular pronoun that goes with innamorarsi.

In very literal terms, Italian treats this as something like I became myself in love, but that literal idea is not how you should translate it. The important point is:

• innamorarsi is the standard verb for to fall in love
• the mi is part of that verb structure

So in this sentence:

• mi sono innamorato di lei = I fell in love with her

You should learn it as a chunk:

• innamorarsi di

The dictionary form is innamorarsi.

That is the infinitive. It normally appears with di:

• innamorarsi di qualcuno = to fall in love with someone
• innamorarsi di qualcosa = to fall in love with something

Examples:

• Mi sono innamorato di lei.
• Si è innamorata di Roma.
• Ci siamo innamorati di questa canzone.

So if you are memorizing it, the best entry is:

• innamorarsi di

Because the past participle agrees with the subject when a verb uses essere in the passato prossimo.

Here, innamorato shows that the speaker is male singular.

• A male speaker says: Mi sono innamorato di lei.
• A female speaker says: Mi sono innamorata di lei.

Plural forms:

• Ci siamo innamorati = a masculine or mixed group
• Ci siamo innamorate = an all-female group

So the ending changes according to who fell in love, not according to lei.

Because innamorarsi requires the preposition di.

So the structure is:

• innamorarsi di qualcuno

That means:

• di lei = of her / with her in the grammar of this expression

Even though English says fall in love with her, Italian uses di here, not a word corresponding directly to with.

More examples:

• Mi sono innamorato di te.
• Si è innamorata di lui.
• Ci siamo innamorati di Maria.

Because after a preposition like di, Italian uses a stressed pronoun, not a direct object pronoun.

So:

• di lei = correct
• di la = incorrect

Compare:

• La amo. = I love her.
  Here la is a direct object pronoun.
• Mi sono innamorato di lei. = I fell in love with her.
  Here lei comes after the preposition di, so you need the stressed form.

Useful set:

• di me
• di te
• di lui
• di lei
• di noi
• di voi
• di loro

Most directly, it means I fell in love with her.

It refers to the moment or process of becoming in love, not just the state.

Compare:

• Mi sono innamorato di lei. = I fell in love with her.
• Sono innamorato di lei. = I am in love with her.

In real usage, the first sentence can also imply the resulting state, but grammatically the focus is on the change: falling in love.

They are related, but they are not the same.

• Mi sono innamorato di lei = I fell in love with her
• La amo = I love her

The first describes becoming in love.
The second describes loving someone.

Also, innamorarsi often sounds more emotional and specific to romantic attachment, while amare can be broader, deeper, or simply a different way of expressing love.

Sometimes someone can say:

• Mi sono innamorato di lei, ma non la amo più.

That would mean they once fell in love, but no longer love her now.

No. It can also be used for places, things, ideas, and experiences, often in a strong or figurative way.

Examples:

• Mi sono innamorato di questa città. = I fell in love with this city.
• Si è innamorata di quel libro. = She fell in love with that book.
• Ci siamo innamorati del panorama. = We fell in love with the view.

So the pattern innamorarsi di is very common even outside romantic contexts.

Yes, absolutely.

If you know the person’s name, you can use it after di:

• Mi sono innamorato di Maria.
• Si è innamorata di Luca.

If the noun uses an article, di combines with it:

• Mi sono innamorato della ragazza.
• Si è innamorata del professore.

So di lei is just one possible complement after innamorarsi.

Yes, but the standard, neutral order is:

• Mi sono innamorato di lei.

You can move parts of the sentence for emphasis, but that makes it more marked or literary:

• Di lei mi sono innamorato.

That sounds more emphatic, like It was her that I fell in love with.

For most learners, the normal order is the safest choice.

Yes, in many contexts you can.

Ne can replace di lei, di lui, di questo, di quella città, and so on, because it stands for something introduced by di.

So:

• Mi sono innamorato di lei.
• Me ne sono innamorato.

The second means I fell in love with her or I fell in love with it, depending on context.

However, there is a difference in tone:

• di lei is more explicit and often more emphatic
• ne is more compact and depends on context

Also notice the pronouns combine:

• mi + ne becomes me ne

To make it negative, put non before the verb phrase:

• Non mi sono innamorato di lei. = I didn’t fall in love with her.

To make it a question, Italian often just uses intonation:

• Ti sei innamorato di lei? = Did you fall in love with her?
• Vi siete innamorati di lei? = Did you fall in love with her? (plural)

You can also add question words:

• Quando ti sei innamorato di lei? = When did you fall in love with her?
• Perché ti sei innamorato di lei? = Why did you fall in love with her?

This is the passato prossimo:

• mi sono
  • innamorato

It is one of the main past tenses in Italian and is commonly used to talk about completed past events, especially in spoken Italian.

Here it presents falling in love as a completed event:

• at some point in the past, this happened

Because innamorarsi uses essere, the structure is:

• pronoun + present of essere
  • past participle

So:

• mi sono innamorato
• ti sei innamorato
• si è innamorata

In many everyday situations, this is exactly the tense an Italian speaker would use.