El testigo vio todo.

El is the definite article, equivalent to the.

In El testigo vio todo, it shows that we are talking about a specific witness, not just any witness in general.

Compare:

• El testigo vio todo = The witness saw everything
• Un testigo vio todo = A witness saw everything

No. Testigo is a noun that can refer to either a male or a female witness.

The gender is usually shown by the article:

• el testigo = a male witness / the male witness
• la testigo = a female witness / the female witness

So the -o ending here does not mean the person must be male.

Vio is the preterite form of the verb ver (to see).

It is:

• third person singular
• used for a completed action in the past

So vio means he saw, she saw, or it saw, depending on context.

In this sentence, it matches el testigo:

• El testigo vio todo = The witness saw everything

The infinitive is ver (to see).

Yes, ver is somewhat irregular, especially in the preterite. Its main preterite forms are:

• vi = I saw
• viste = you saw
• vio = he/she saw
• vimos = we saw
• visteis = you all saw
• vieron = they saw

A learner often expects something more regular, but ver has to be memorised.

Because ve is present tense, while vio is past tense.

Compare:

• El testigo ve todo = The witness sees everything
• El testigo vio todo = The witness saw everything

So vio tells you the action happened in the past.

Here todo means everything, so it is being used as a pronoun, not as an adjective describing a noun.

That is why it stays in the singular form todo.

Compare:

• vio todo = saw everything
• vio todos los libros = saw all the books
• vio toda la película = saw the whole film

So in your sentence, todo does not mean all + noun. It means everything by itself.

It is a pronoun here.

In El testigo vio todo, todo stands alone and means everything.

If it were an adjective, it would go with a noun:

• todo el día = all day / the whole day
• toda la casa = the whole house
• todos los testigos = all the witnesses

Since there is no noun after todo in your sentence, it is acting as a pronoun.

Because vio is treated as one syllable in standard Spanish, and one-syllable words normally do not take a written accent.

So it is written:

• vio ✅
• vió ❌

This often surprises learners because it may look like it should have two syllables, but in standard spelling it does not.

In standard Spanish, vio is usually pronounced as one syllable, roughly like byo.

A few useful points:

• the v sound in Spanish is pronounced very similarly to b
• io here forms one syllable
• so vio is not normally pronounced as vee-oh

A rough guide:

• vio ≈ byo

The normal, neutral order here is:

Subject + verb + object

• El testigo vio todo

Spanish word order is more flexible than English, though. Other orders are possible, but they usually sound more marked or are used for emphasis.

For example:

• Vio todo el testigo = possible in some contexts, but not the most neutral order
• Todo lo vio el testigo = more emphatic, something like It was the witness who saw it all

For a learner, El testigo vio todo is the most straightforward version.

Yes. Spanish often drops the subject when it is clear from context.

So:

• Vio todo = He/She saw everything

Because the verb form vio already tells you it is third person singular, Spanish does not always need the subject pronoun or noun.

However, in your sentence, el testigo is included to make it clear exactly who did the action.

Because todo here is a direct object meaning everything, not a specific person.

Spanish uses the personal a mainly before a specific human direct object:

• Vio al testigo = He/She saw the witness

But:

• Vio todo = He/She saw everything
• Vio la casa = He/She saw the house

So there is no a because todo is not a person.