Lo olvidé completamente.

Lo is a direct object pronoun meaning it (or sometimes him, in some contexts, though here learners usually meet it as it).

In this sentence, it stands for the thing that was forgotten:

• Olvidé el libro. = I forgot the book.
• Lo olvidé. = I forgot it.

So lo replaces a previously mentioned masculine singular noun, or sometimes an idea/situation treated as it.

In Spanish, unstressed object pronouns normally go before a conjugated verb.

So:

• Lo olvidé = I forgot it

not:

• Olvidé lo ❌

Pronouns can attach to the end of the verb only in certain structures, such as:

• olvidarlo = to forget it
• olvídalo = forget it

But with a normal conjugated past-tense verb like olvidé, the pronoun goes before it.

Olvidé is the first-person singular preterite form of olvidar:

• olvidar = to forget
• olvidé = I forgot

This tense is used for a completed action in the past.

So Lo olvidé completamente means that the forgetting happened and is viewed as finished.

The accent mark shows both stress and helps distinguish the form.

• olvidé = I forgot
• olvide = a different form or a misspelling in this context

In olvidé, the stress falls on the last syllable: ol-vi-DÉ.

That accent is important in writing because it marks the correct preterite ending for yo in many -ar verbs:

• hablé
• compré
• olvidé

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

• olvidé already means I forgot

So:

• Lo olvidé completamente. = natural
• Yo lo olvidé completamente. = also correct

Adding yo is usually for emphasis, contrast, or clarity:

• Yo lo olvidé, no ella. = I forgot it, not her.

The direct object pronoun would change to match the noun:

• Lo olvidé = I forgot it (masculine singular)
• La olvidé = I forgot it (feminine singular)
• Los olvidé = I forgot them (masculine/mixed plural)
• Las olvidé = I forgot them (feminine plural)

Examples:

• Olvidé la llave. → La olvidé.
• Olvidé los documentos. → Los olvidé.

Yes. Lo olvidé completamente is grammatical and natural.

Completamente is an adverb meaning completely, and here it modifies olvidé.

Other natural possibilities include:

• Lo olvidé por completo.
• Lo olvidé completamente.

Both are common. Por completo often sounds a little more idiomatic in everyday Spanish, but completamente is perfectly fine.

This is a very common question.

• Lo olvidé = I forgot it
• Me olvidé by itself usually means I forgot / it slipped my mind, but it often needs more context or a complement.

Very often Spanish speakers say:

• Me olvidé de eso. = I forgot about that.
• Se me olvidó. = I forgot it / It slipped my mind.

So the patterns are slightly different:

1. olvidar + direct object

   • Lo olvidé.

2. olvidarse de + noun/pronoun

   • Me olvidé de eso.

3. Very common spontaneous-forgetting structure:

   • Se me olvidó.

In your sentence, Lo olvidé completamente is a straightforward, correct way to say I completely forgot it.

Yes. Lo can refer to a whole idea, fact, or situation that has already been mentioned.

For example:

• —¿Recuerdas que teníamos una cita?
• —No, lo olvidé completamente.

Here lo does not refer to a single physical object. It refers to the fact that we had an appointment.

This is very common in Spanish.

No, not exactly.

• Lo olvidé completamente = I completely forgot it
  
  • a completed past action
• Lo olvidaba completamente = I was completely forgetting it / I used to completely forget it
  
  • imperfect, which gives a different sense

The preterite (olvidé) is the natural choice when you mean a specific completed act of forgetting.

A simple guide for Spain Spanish:

• Lo → like loh
• olvidé → ol-vee-DEH
• completamente → kom-pleh-ta-MEN-teh

Approximate full pronunciation:

loh ol-vee-DEH kom-pleh-ta-MEN-teh

A few notes:

• The v in Spanish is pronounced much like b.
• The t is a clear dental t, softer than in English.
• In Spain, the c in completamente is a normal k sound because it is before o.

Yes. That would also be correct, but it is a different tense.

• Lo olvidé completamente = preterite; a completed past action
• Lo he olvidado completamente = present perfect; often connected more to the present

In Spain, the present perfect is used quite a lot for recent past actions or past actions felt as relevant now. So in Spain Spanish, Lo he olvidado completamente can sound very natural in the right context.

It is neutral. It works in both formal and informal situations.

Nothing in Lo olvidé completamente is especially casual, slangy, or formal. It is just standard Spanish.

If you wanted a slightly more conversational alternative, you might also hear:

• Se me olvidó por completo.

But your sentence is perfectly standard and natural.