यह दवा जल्दी असर करेगी।

Because दवा (medicine) is a feminine singular noun, and the future form of करना (to do) has to agree with it here.

• masculine singular: करेगा
• feminine singular: करेगी
• plural / respectful: करेंगे

So:

• यह दवा असर करेगी = This medicine will have an effect
• यह इलाज असर करेगा = This treatment will have an effect

असर is a noun meaning effect, and करना means to do. Together, असर करना is a very common Hindi expression meaning:

• to have an effect
• to work
• to take effect

So यह दवा जल्दी असर करेगी literally feels like:

This medicine will do effect quickly

But in natural English, you would usually say:

• This medicine will work quickly
• This medicine will take effect quickly

It can suggest either, depending on context, and in this sentence both ideas are close.

Here जल्दी most naturally means:

• quickly
• soon

So the sentence can imply:

• the medicine will begin working soon
• the medicine will take effect quickly

Hindi जल्दी often covers both quickly and early/soon, so context matters.

In standard written Hindi, यह is the singular form meaning this.

So:

• यह दवा = this medicine
• ये दवाएँ = these medicines

In everyday spoken Hindi, many people say ये even for singular this, so you may hear:

• ये दवा जल्दी असर करेगी

But in careful standard Hindi, यह दवा is the expected form.

Because ने is mainly used with the perfective past in Hindi, not with the simple future.

This sentence is in the future:

• यह दवा जल्दी असर करेगी = This medicine will take effect quickly

Compare with a past sentence:

• इस दवा ने जल्दी असर किया = This medicine had an effect quickly / worked quickly

So no ने is needed in the future version.

असर is generally treated as a masculine noun. But in this sentence, the form करेगी is not agreeing with असर. It is agreeing with दवा, the subject.

That is why we get:

• यह दवा असर करेगी

not because असर is feminine, but because दवा is feminine.

The order is very natural for Hindi:

• यह = this
• दवा = medicine
• जल्दी = quickly/soon
• असर करेगी = will have an effect / will work

So the structure is roughly:

this medicine quickly effect will-do

Hindi usually puts the main verb at the end, and adverbs like जल्दी often come before the verb phrase.

It is understandable, but यह दवा जल्दी असर करेगी sounds more natural.

Placing जल्दी before असर करेगी is the most normal choice. Hindi adverbs are somewhat flexible, but some word orders sound smoother and more idiomatic than others.

Most natural:

• यह दवा जल्दी असर करेगी

Less natural but understandable:

• यह दवा असर जल्दी करेगी

Yes, and learners often hear that too.

• असर करना = to have an effect / take effect
• काम करना = to work

So:

• यह दवा जल्दी असर करेगी
• यह दवा जल्दी काम करेगी

Both can be used, but असर करेगी is especially common when talking about medicine, treatment, or something having an effect on the body.

A simple pronunciation guide is:

yah da-vaa jal-dee a-sar ka-ray-gee

A more careful transliteration would be:

yah davā jaldī asar karegī

A few helpful notes:

• यह is often pronounced yah
• दवा has two syllables: da-vā
• जल्दी sounds like jal-dee
• करेगी sounds like ka-ray-gee

Both are natural, but they focus on slightly different things.

1. यह दवा जल्दी असर करेगी

   • This medicine will take effect quickly
   • Focuses on the medicine doing the affecting

2. इस दवा का असर जल्दी होगा

   • The effect of this medicine will come quickly
   • Focuses more on the effect itself

Both are correct, and both are common in medical contexts.

You would need to make both the demonstrative and the verb plural:

• ये दवाएँ जल्दी असर करेंगी।

Breakdown:

• ये = these
• दवाएँ = medicines
• करेंगी = will do / will have effect (feminine plural)

So singular:

• यह दवा जल्दी असर करेगी।

Plural:

• ये दवाएँ जल्दी असर करेंगी।