Vimos un rayo en la tormenta.

Vimos is the preterite form of ver (to see) for we.

• ver = to see
• vimos = we saw

The preterite is used for a completed action in the past, so this sentence means that the sighting happened as a finished event.

It comes from ver, which is only slightly irregular in the preterite.

The preterite forms are:

• vi = I saw
• viste = you saw
• vio = he/she/it saw
• vimos = we saw
• visteis = you all saw (used in Spain)
• vieron = they saw

What is unusual is that these forms do not take written accents.

Spanish often leaves out subject pronouns when the verb already shows who is doing the action.

So:

• Vimos un rayo = We saw a lightning bolt
• Nosotros vimos un rayo = also correct, but more emphatic

You would usually add nosotros only if you want contrast or emphasis, such as:

• Nosotros vimos un rayo, pero ellos no.
  We saw a lightning bolt, but they didn’t.

Un rayo means a lightning bolt or a flash of lightning in a general, indefinite sense.

• un rayo = a lightning bolt
• el rayo = the lightning bolt

You use un when introducing something not already identified.
You would use el if both speaker and listener already know which one you mean.

Here, rayo means lightning or a bolt of lightning.

A useful weather distinction is:

• rayo = lightning bolt / strike
• relámpago = flash of lightning
• trueno = thunder

In everyday Spanish, people may sometimes use these loosely, but this is the basic distinction.

Here en la tormenta means in the storm or during the storm.

With weather and events, en often has a broader sense than literal physical location. So it can mean:

• during the storm
• in the middle of the storm
• as part of the storm situation

A close alternative would be durante la tormenta (during the storm), but en la tormenta sounds natural.

Yes. That is also correct.

• en la tormenta = in the storm / during the storm
• durante la tormenta = during the storm

Durante makes the time relationship more explicit.
En la tormenta can sound a little more scene-based, as if placing the event within the storm itself.

The personal a is normally used before a specific human person, and sometimes pets or personified beings.

Since un rayo is a thing, not a person, there is no personal a:

• Vimos un rayo. = correct
• Vimos a María. = correct, because María is a person

Yes, but the original order is the most neutral:

• Vimos un rayo en la tormenta.

You might also hear:

• En la tormenta vimos un rayo.

This puts more focus on the storm or sets the scene first. Both are grammatical.

You know it from the verb form and context.

Vimos can only be first person plural in the preterite of ver:

• vimos = we saw

Spanish verb endings often carry the subject information, which is why the pronoun is usually omitted.

Yes, it is natural and correct.

A native speaker might also say:

• Vimos un relámpago durante la tormenta.
• Vimos caer un rayo durante la tormenta.
• Durante la tormenta vimos un rayo.

But Vimos un rayo en la tormenta is perfectly understandable and normal.

In most of Spain, rayo is pronounced roughly like RAH-yo.

A few pronunciation notes:

• r at the beginning of a syllable is a tapped or trilled Spanish r
• y in rayo is usually pronounced like an English y sound
• ayo sounds like ah-yo

So a simple learner-friendly approximation is RAH-yo.

Because the preterite forms of ver are written without accents:

• vi
• viste
• vio
• vimos
• visteis
• vieron

This surprises learners because many past-tense forms in Spanish do have written accents, but ver in the preterite does not.