मैंने जल्दी जवाब लिखा।

Because ने is the postposition often used with the subject in the perfective past when the verb is transitive.

• मैं = I
• मैंने = I
  • ने

In sentences like this, Hindi commonly says:

• मैंने किताब पढ़ी। = I read the book.
• मैंने जवाब लिखा। = I wrote the reply.

This is part of Hindi’s ergative pattern in the past/perfective. English does not do this, so it feels unusual at first.

ने marks the doer of the action in many perfective transitive sentences.

Here:

• मैंने = I as the one who did the writing

A very important point: when ने is used, the verb often agrees with the direct object, not with the subject.

That is why learners often pay attention to:

1. whether the verb is transitive,
2. whether the sentence is perfective,
3. what gender/number the object has.

The form लिखा is masculine singular, and it agrees with जवाब, which is a masculine singular noun.

So:

• जवाब = masculine singular
• therefore लिखा

Compare:

• मैंने जवाब लिखा। = I wrote the reply.
• मैंने चिट्ठी लिखी। = I wrote the letter.

Why लिखी in the second sentence? Because चिट्ठी is feminine.

Yes, that is the key idea in this sentence.

With ने in a perfective transitive clause, the verb typically agrees with the direct object if that object is not blocked by another marker.

So in:

• मैंने जवाब लिखा।

the verb agrees with जवाब.

But if the object is marked with को, agreement often defaults differently. For example:

• मैंने उसको देखा।

Here the object is marked, so the agreement pattern changes.

For a beginner, the useful takeaway is:

• ने often stops the verb from agreeing with the subject.
• The verb may then agree with the direct object instead.

जल्दी is an adverb, meaning it describes how the action happened: quickly.

Hindi word order is flexible, but adverbs often come before the verb phrase:

• मैंने जल्दी जवाब लिखा।

This sounds natural and means something like I wrote the reply quickly.

You may also hear variations depending on emphasis, such as:

• मैंने जवाब जल्दी लिखा।

Both are possible, but the original sentence is very natural.

It can mean either, depending on context.

Common uses:

• quickly
• soon
• early

In this sentence, it most naturally means quickly.

Examples:

• जल्दी लिखो। = Write quickly.
• जल्दी आओ। = Come soon / come quickly.
• मैं जल्दी आया। = I came early.

So the exact meaning comes from context.

Yes. जल्द and जल्दी are both used, and both can mean quickly/soon.

• मैंने जल्दी जवाब लिखा।
• मैंने जल्द जवाब लिखा।

Both are understandable. In everyday speech, जल्दी is often more common and may feel a bit more natural to many learners at first.

They are similar, but not always used in exactly the same way.

• जवाब is very common in everyday speech and often means answer or reply.
• उत्तर can also mean answer, but it often sounds more formal or academic in many contexts.

For example:

• सवाल का जवाब = answer to a question
• पत्र का जवाब = reply to a letter
• प्रश्न का उत्तर = answer to a question (more formal/bookish)

In your sentence, जवाब sounds very natural.

Yes. जवाब can mean both answer and reply, depending on context.

So the sentence could be understood as:

• I wrote the answer quickly
• I wrote the reply quickly

If the wider context is an exam or a question, answer is likely. If the context is a message, letter, or email, reply is likely.

Yes, very often.

Because the verb form and context may make the meaning clear, Hindi can omit the subject:

• जल्दी जवाब लिखा।

In conversation, this could mean I wrote the reply quickly, if the context already tells us who did it.

But the full sentence:

• मैंने जल्दी जवाब लिखा।

is clearer and very useful for learners.

No. That changes the tense/aspect completely.

• मैंने जल्दी जवाब लिखा। = I wrote the reply quickly.
  
  • completed past action
• मैं जल्दी जवाब लिखता हूँ। = I write replies quickly / I tend to write replies quickly.
  
  • habitual

So लिखा here is not just a different ending; it expresses a completed action in the past.

Yes, it is natural and grammatical.

It is a straightforward sentence with:

• subject: मैंने
• adverb: जल्दी
• object: जवाब
• verb: लिखा

A native speaker might also choose slightly different phrasing depending on context, such as:

• मैंने जवाब जल्दी लिखा।
• मैंने जल्दी से जवाब लिखा।

But your sentence is perfectly normal Hindi.