Ti assicuro che tutto è pronto.

Because ti and tu do different jobs.

• tu = you as the subject of the verb
• ti = you as the object of the verb

In Ti assicuro, the subject is understood as I:

• (Io) ti assicuro = I assure you

So ti means you in I assure you, not you assure.

Italian often leaves out subject pronouns like io, because the verb already shows who is doing the action.

Assicuro is the 1st person singular present of assicurare.

Here it means:

• I assure
• I reassure (depending on context)

So:

• Ti assicuro che... = I assure you that...

It is a very common way to emphasize sincerity or certainty.

Che here means that and introduces a subordinate clause.

• Ti assicuro che tutto è pronto.
• I assure you that everything is ready.

In English, that is often optional:

• I assure you everything is ready

In Italian, che is normally kept in this structure.

Because tutto here is treated as a singular idea: everything, all of it.

So Italian uses singular agreement:

• tutto è pronto = everything is ready

Compare:

• Tutto è pronto. = Everything is ready.
• Tutti sono pronti. = Everyone / all of them are ready.

So:

• tutto = everything, all of it → singular
• tutti = everyone / all of them → plural

Because the subject is tutto, which is singular.

So the verb and adjective must agree with that singular subject:

• tutto è pronto

Breakdown:

• è = is
• pronto = ready (masculine singular)

Italian agreement follows the grammatical form of the subject, not always the English idea behind it.

Because pronto agrees with tutto, which is masculine singular.

Agreement patterns:

• masculine singular: pronto
• feminine singular: pronta
• masculine plural: pronti
• feminine plural: pronte

Examples:

• Tutto è pronto. = Everything is ready.
• La cena è pronta. = Dinner is ready.
• I documenti sono pronti. = The documents are ready.

Because è is the verb is (from essere), and the accent distinguishes it from e, which means and.

• è = is
• e = and

So:

• Tutto è pronto = Everything is ready
• pane e formaggio = bread and cheese

This accent is very important in writing.

Yes. That is completely correct.

• Ti assicuro che tutto è pronto.
• Io ti assicuro che tutto è pronto.

The version without io is more neutral and more common, because Italian usually drops subject pronouns when they are clear from the verb.

Adding io can give emphasis, as in:

• I assure you...

Yes, to a degree.

The most neutral order is:

• Ti assicuro che tutto è pronto.

But Italian can move parts around for emphasis:

• Tutto è pronto, ti assicuro.
• Ti assicuro: tutto è pronto.

These alternatives are possible, but the original sentence is the most straightforward and standard.

You would usually say:

• Le assicuro che tutto è pronto.

Here:

• Le = formal you object pronoun
• assicuro = I assure

So:

• Ti assicuro... = informal, singular you
• Le assicuro... = formal, singular you

In speech, the formal pronoun is based on Lei, but object forms like Le are commonly used in this kind of sentence.

Yes. Ti assicuro adds emphasis and personal reassurance.

Compare:

• Tutto è pronto. = Everything is ready.
• Ti assicuro che tutto è pronto. = I assure you that everything is ready.

The second one suggests:

• the speaker wants to convince the listener
• there may have been doubt, worry, or impatience
• the speaker is stressing certainty

Sometimes, yes, but the tone changes.

• Ti assicuro che tutto è pronto = I assure you that everything is ready
• Ti garantisco che tutto è pronto = I guarantee you that everything is ready

Garantire often sounds stronger, more formal, or more absolute.
Assicurare is usually the more natural everyday choice for reassuring someone.

Yes. Ti is an unstressed object pronoun, and in normal finite verb forms it usually comes before the verb.

So:

• Ti assicuro = I assure you

This is part of a general pattern:

• Ti vedo = I see you
• Ti chiamo = I call you
• Ti dico = I tell you

With infinitives, gerunds, and affirmative commands, pronouns can attach to the end instead:

• assicurarti = to assure you
• assicurandoti = assuring you
• assicurami = assure me

But in your sentence, ti goes before assicuro.