El rayo iluminó el cielo.

Here, rayo means a bolt of lightning.

A few related words are easy to confuse:

• rayo = a lightning bolt; also a ray/beam
• relámpago = lightning/the flash of lightning
• trueno = thunder

So in this sentence, el rayo is the thing that lit up the sky.

Both are possible, but they feel slightly different.

• El rayo can refer to a specific or contextually understood lightning bolt, or it can sound more vivid/literary.
• Un rayo would mean a lightning bolt, introducing it more neutrally.

So:

• El rayo iluminó el cielo = the lightning bolt lit up the sky
• Un rayo iluminó el cielo = a lightning bolt lit up the sky

Spanish often uses the definite article in places where English might be less specific.

Because el here is the masculine singular definite article: the.

Compare:

• el = the
• él = he

So El rayo means the lightning bolt, not he lightning bolt.

Iluminó is the third-person singular preterite of iluminar.

That means it matches a subject like:

• él = he
• ella = she
• ello = it
• el rayo = the lightning bolt

It describes a completed action in the past: the lightning bolt lit up the sky.

Because iluminó presents the action as a single completed event.

• iluminó = it lit up / it illuminated
• iluminaba = it was lighting up / it used to light up

With a sudden event like lightning, the preterite iluminó is the natural choice if you are describing one flash.

The accent mark shows that the stress falls on the last syllable: i-lu-mi-NÓ.

It also helps distinguish this form from other verb forms. For example:

• ilumino = I illuminate / I do illuminate
• iluminó = he/she/it illuminated

So the accent is important for both pronunciation and meaning.

Because Spanish usually needs an article here.

In English, we often say the sky, and Spanish does the same: el cielo.

Saying just cielo by itself would usually sound incomplete in this sentence. The normal form is:

• iluminó el cielo

This is simply how Spanish handles nouns in this kind of context.

Because el cielo is a direct object, and it is not a person.

Spanish uses the personal a mainly with specific human direct objects:

• Veo a María
• Escucho a mi profesor

But with things, you normally do not use a:

• iluminó el cielo
• vio la casa
• abrió la puerta

So a is not needed here.

Yes.

Cielo can mean:

• sky
• heaven

The context tells you which meaning is intended. In El rayo iluminó el cielo, the natural meaning is sky.

A common Spain pronunciation is roughly:

el RRA-yo ee-loo-mee-NO el THYE-lo

More carefully:

• el
• rayo = RRA-yo
  
  • the r at the start is strong/trilled
• iluminó = ee-loo-mee-NO
• cielo = THYE-lo in most of Spain

A few useful pronunciation notes:

• c before i in most of Spain sounds like th in think
• y in rayo usually sounds like an English y
• the stress is on -nó in iluminó

Yes, but El rayo iluminó el cielo is the most neutral and standard order: subject + verb + object.

Other orders are possible for emphasis or style, especially in writing:

• Iluminó el cielo el rayo
• El cielo lo iluminó el rayo (with extra emphasis and a different structure)

But for a learner, the original sentence is the best normal pattern to follow.

This is a very common vocabulary question.

• rayo = a lightning bolt
• relámpago = the flash of lightning
• trueno = thunder, the sound

So:

• You see a relámpago
• A rayo can strike
• You hear a trueno

In everyday speech, people do sometimes use these words a bit loosely, but that is the basic distinction.