Hablé con mi nieto.

Because hablé is in the past tense.

• hablo = I speak / I am speaking
• hablé = I spoke / I talked

In this sentence, the speaker is referring to a completed action in the past, so Spanish uses hablé, the yo form of the pretérito (preterite).

Hablé is the first-person singular preterite of hablar.

That means:

• hablar = to speak / to talk
• hablé = I spoke / I talked

The preterite is commonly used for a finished action in the past, especially something seen as complete.

Spanish often leaves out subject pronouns because the verb ending already tells you who the subject is.

• hablé already means I spoke
• so yo hablé is usually unnecessary

You can add yo for emphasis, contrast, or clarity:

• Yo hablé con mi nieto, no con mi hija.
  I spoke with my grandson, not with my daughter.

So Hablé con mi nieto is perfectly natural.

Because hablar con alguien means to talk with someone or very often to talk to someone.

So:

• hablé con mi nieto = I spoke with my grandson / I talked to my grandson

Spanish commonly uses con where English might say either with or to, depending on the context.

Sometimes, but it is not the most natural choice here.

• hablar con alguien suggests a conversation or interaction
• hablar a alguien can sound more like speaking to someone in a one-way sense

In most everyday situations, if you mean you had a conversation with your grandson, hablé con mi nieto is the normal phrasing.

Because mi nieto is not the direct object of the verb here. It comes after the preposition con.

The personal a is used before a direct object that is a person:

• Vi a mi nieto. = I saw my grandson.

But here the structure is:

• hablé con mi nieto

Since mi nieto is part of the prepositional phrase with con, there is no extra a.

These are two different words.

• mi = my
• mí = me after a preposition

So in this sentence:

• mi nieto = my grandson

Compare:

• para mí = for me
• conmigo = with me

No accent is used in mi when it means my.

Nieto means grandson.

It is:

• masculine
• singular

Related forms:

• nieta = granddaughter
• nietos = grandchildren / grandsons
• nietas = granddaughters

So mi nieto specifically means my grandson.

The accent mark shows where the stress falls and also helps distinguish this form from others.

• hablé = preterite, I spoke
• hable = present subjunctive or formal command form, depending on context

With hablé, the stress is on the last syllable: ha-BLÉ.

That accent is important both for pronunciation and meaning.

It can mean either, depending on context.

• I spoke with my grandson
• I talked with my grandson
• I talked to my grandson

All are reasonable translations of Hablé con mi nieto. Spanish hablar often covers both speak and talk.

A common Spain pronunciation would be roughly:

a-BLÉ con mi NYE-to

A few useful points:

• h is silent in hablé
• bl is pronounced clearly together
• é is stressed
• con has a clear k sound at the start
• nieto begins with nie-, which sounds like nye-

In IPA, a common pronunciation is approximately:

[aˈβle kon mi ˈnjeto]

In some contexts, an English translation like I have spoken with my grandson may fit, but grammatically hablé is specifically the simple past in Spanish.

The more direct Spanish equivalent of I have spoken is:

• He hablado con mi nieto

That said, translation depends on context, and English sometimes uses different past forms where Spanish uses the preterite.

In the preterite, hablar goes:

• yo hablé = I spoke
• tú hablaste = you spoke
• él / ella / usted habló = he/she/you spoke
• nosotros hablamos = we spoke
• vosotros hablasteis = you all spoke
• ellos / ellas / ustedes hablaron = they/you all spoke

Your sentence uses the first one: yo hablé.