Non dimenticare di obliterare anche il mio biglietto, perché io ho le mani occupate.

Because this is a negative command addressed to tu. In Italian, the negative imperative for tu is:

non + infinitive

So:

• Non dimenticare = Don’t forget
• Non parlare = Don’t speak
• Non andare = Don’t go

By contrast, dimentica would be the positive informal command: Forget!

After dimenticare, Italian normally uses di + infinitive when you mean to forget to do something.

So:

• dimenticare di fare qualcosa = to forget to do something

Examples:

• Ho dimenticato di chiamarti = I forgot to call you
• Non dimenticare di chiudere la porta = Don’t forget to close the door

So Non dimenticare di obliterare... means Don’t forget to validate/stamp...

Not here. This is a very important false friend.

In everyday transport Italian, obliterare un biglietto means to validate, stamp, punch, or cancel a ticket before using it.

So in this sentence:

• obliterare il biglietto = to validate/stamp the ticket

It does not mean destroy the ticket.

Depending on region or context, you may also hear:

• convalidare il biglietto
• timbrare il biglietto
• validare il biglietto

It is informal singular, because Non dimenticare is a command to tu.

Other versions would be:

• Informal singular: Non dimenticare di obliterare...
• Formal singular: Non dimentichi di obliterare...
• Plural: Non dimenticate di obliterare...

So the speaker is talking to one person in an informal way.

Because anche usually goes right before the word or phrase it is focusing on.

Here:

• obliterare anche il mio biglietto = validate my ticket too / also my ticket

The emphasis is on il mio biglietto: not just some other ticket, but mine as well.

If you move anche, the emphasis can change slightly, although not all placements sound equally natural.

Italian often leaves subject pronouns out, so perché ho le mani occupate would be perfectly normal.

The pronoun io is added for emphasis or contrast.

So:

• perché ho le mani occupate = neutral, natural
• perché io ho le mani occupate = because I have my hands full, with extra stress on I

It can sound like:

• because I’m the one who has my hands occupied
• because I have my hands full

With body parts, Italian often uses the definite article instead of a possessive adjective when the owner is already clear.

So Italian prefers:

• ho le mani occupate = literally I have the hands occupied, but naturally my hands are occupied / I have my hands full

This is very common with body parts:

• Mi lavo le mani = I wash my hands
• Ha chiuso gli occhi = He/She closed his/her eyes
• Mi fa male la testa = My head hurts

You can say le mie mani, but it is usually unnecessary here.

Because occupate agrees with mani.

• mano = feminine singular
• mani = feminine plural

So the adjective must also be feminine plural:

• la mano occupata
• le mani occupate

This agreement is a basic Italian rule: adjectives match the noun in gender and number.

Not exactly.

• Ho le mani occupate means my hands are occupied / I have my hands full, often literally because I’m carrying something.
• Sono occupato/a means I’m busy in a more general sense.

So in this sentence, ho le mani occupate is more precise: the person cannot easily stamp the ticket because their hands are not free.

Here biglietto means a travel ticket, such as a bus, tram, or train ticket.

Italian biglietto can mean different kinds of ticket depending on context:

• biglietto del treno = train ticket
• biglietto dell’autobus = bus ticket
• biglietto per il concerto = concert ticket

In this sentence, because of obliterare, it clearly refers to a ticket that must be validated before use.