Respondió inmediatamente.

Respondió is in the preterite tense, which is used for a completed action in the past.

So Respondió inmediatamente describes one finished event: the person answered/replied immediately.

Compare:

• respondió = answered / replied
• respondía = was answering / used to answer

By itself, respondió can mean:

• he answered
• she answered
• you answered (formal singular, usted)

Spanish often leaves out the subject pronoun because the verb ending already gives that information. So the exact subject depends on the context.

If needed, you could make it explicit:

• Él respondió inmediatamente.
• Ella respondió inmediatamente.
• Usted respondió inmediatamente.

The accent mark in respondió shows where the stress goes: on the final syllable -dió.

So it is pronounced roughly like:

• res-pon-DYO

The accent is also important because it matches the standard written form of this preterite ending in Spanish.

Yes. Responder is a regular -er verb in the preterite.

Its preterite forms are:

• yo respondí
• tú respondiste
• él/ella/usted respondió
• nosotros respondimos
• vosotros respondisteis
• ellos/ellas/ustedes respondieron

So respondió follows the normal pattern for regular -er verbs.

It can mean either one, depending on context.

Responder is commonly used for:

• answering a question
• replying to a person, message, or comment

So Respondió inmediatamente could mean:

• He/She answered immediately
• He/She replied immediately

Spanish often uses the same verb where English might choose between answer and reply.

You can add it, but in normal Spanish it is often left out unless you want emphasis, contrast, or clarity.

Natural:

• Respondió inmediatamente.

Also possible:

• Él respondió inmediatamente.

You would add the pronoun if:

• you want to contrast people: Ella respondió inmediatamente, pero él no.
• the subject is unclear
• you want emphasis

So in many cases, no pronoun sounds more natural.

That is the most neutral and natural word order here:

• Respondió inmediatamente.

Spanish adverbs like inmediatamente often come after the verb, especially in simple statements.

But other positions are also possible:

• Inmediatamente respondió.
  This sounds more literary or emphatic.
• Respondió, inmediatamente, ...
  Possible in certain spoken or written styles, but less neutral here.

So the original sentence uses the most standard order.

Yes. Inmediatamente is completely correct, but in everyday speech in Spain, people often also use:

• enseguida = right away
• al momento = immediately / right away
• de inmediato = immediately

Examples:

• Respondió enseguida.
• Respondió al momento.

Inmediatamente is slightly more formal or neutral, while enseguida is very common in everyday conversation.

A simple pronunciation guide:

• Respondió ≈ reh-spon-DYO
• inmediatamente ≈ een-meh-dya-ta-MEN-teh

A few helpful points:

• Spanish r in respondió is a single tap here, not a strong English r
• dió sounds like dyo
• j does not appear here, so there is no harsh sound to worry about
• stress falls on:
  • res-pon-DIÓ
  • in-me-dya-ta-MEN-te

Often, yes.

• respondió = answered / replied
• contestó = answered / replied

In many contexts they are interchangeable:

• Respondió inmediatamente.
• Contestó inmediatamente.

However, usage can vary a bit by region and situation. Responder is a very safe, general verb. Contestar is also common, including in Spain.

One small note for learners: in English, to contest means something different, but Spanish contestar usually means to answer, not to dispute.

Because Spanish, like English, can leave the object understood from context.

For example, if everyone already knows what was being answered, you can simply say:

• Respondió inmediatamente.

The missing object might be something like:

• Respondió la pregunta.
• Respondió al mensaje.
• Respondió a su correo.

But if context already makes this clear, Spanish often leaves it out.

This is a very common question because it is the difference between the preterite and the imperfect.

• respondió = he/she answered (completed event)
• respondía = he/she was answering / used to answer / would answer

So:

• Respondió inmediatamente.
  = a single completed action
• Respondía inmediatamente cuando le escribían.
  = a habitual action in the past

In your sentence, the preterite is used because the action is seen as one finished event.

Yes, it is perfectly natural and correct in Spain.

It sounds:

• clear
• standard
• neutral

In casual speech, a person in Spain might also say:

• Respondió enseguida.
• Contestó enseguida.

But Respondió inmediatamente is completely normal and natural Spanish.