यह गोली खाना खाने के बाद लेनी है।

Because the two forms are doing different jobs.

• The first खाना means food / a meal.
• The second खाने comes from the verb खाना = to eat, and here it means eating in the phrase के बाद = after.

So खाना खाने के बाद literally feels like after eating food.

In natural English, you would usually just say after eating or after a meal, but Hindi often uses this fuller expression very naturally.

Yes. खाने के बाद is also correct and very common.

So both can work:

• खाने के बाद = after eating
• खाना खाने के बाद = after eating food / after having a meal

The longer version can sound a little more explicit. In medicine instructions, it often suggests after a proper meal, not just after tasting something small.

यह means this, and गोली means pill or tablet.

So:

• यह = this
• गोली = pill

Together, यह गोली = this pill.

Because गोली is a feminine noun in Hindi, and लेनी agrees with it.

• गोली is feminine singular
• so the form is लेनी

Compare:

• यह गोली लेनी है। = This pill has to be taken.
• यह दवा लेनी है। = This medicine has to be taken.
  (if the noun is feminine)
• यह सिरप लेना है। = This syrup has to be taken.
  (if the noun is masculine)

So the -नी ending is there because the thing being taken, गोली, is feminine.

Not exactly. Here लेनी है expresses something like:

• has to be taken
• should be taken
• is to be taken

So the whole sentence is not just describing an action. It is giving an instruction.

A very literal feel is:

• This pill is to be taken after food.

So लेनी है often carries a sense of requirement or instruction, not just simple present tense.

In Hindi, medicine is very often spoken of with लेना = to take.

So:

• दवा लेना = to take medicine
• गोली लेना = to take a pill

This matches English quite well, because English also says take medicine.

You may also hear गोली खाना, especially in everyday speech, because pills are swallowed. But in instructions, लेना is very common and often sounds more standard or neutral.

It is understood, not stated.

Hindi often leaves out pronouns when the meaning is obvious from context. In a medicine instruction, the listener understands that it means:

• You should take this pill after food or
• This pill should be taken after food

So Hindi does not need to say तुम, आप, or तू here.

के बाद means after.

In this sentence, it follows खाने:

• खाने के बाद = after eating

The word बाद usually works with के after a noun or verbal noun:

• खाने के बाद = after eating
• स्कूल के बाद = after school
• काम के बाद = after work

So it is best to learn के बाद as a set phrase meaning after.

Hindi normally puts the main verb near the end of the sentence.

So the structure is roughly:

• यह गोली = this pill
• खाना खाने के बाद = after eating food
• लेनी है = has to be taken

That gives:

• This pill after eating food has to be taken

That sounds odd in English, but it is normal in Hindi. Time phrases and other extra information often come before the main verb.

For a learner, it is easiest to think of it as an instructional construction meaning has to be taken.

It is not the most straightforward active sentence like:

• आप यह गोली लेते हैं। = You take this pill.

And it is also not the full formal passive you might see as:

• यह गोली खाना खाने के बाद ली जानी है।

Your sentence, यह गोली खाना खाने के बाद लेनी है, is a very common and natural way to give instructions. It is often the most idiomatic choice in everyday Hindi.

It can mean food in general, but in context it often means a meal.

So in a medicine instruction, खाना खाने के बाद is usually understood as:

• after eating a meal
• after food
• after meals

The exact meal is not specified unless another word is added, such as:

• नाश्ते के बाद = after breakfast
• दोपहर के खाने के बाद = after lunch
• रात के खाने के बाद = after dinner

Yes, the form would change to match the plural noun.

For example:

• यह गोली लेनी है। = This pill has to be taken.
• ये गोलियाँ लेनी हैं। = These pills have to be taken.

Notice two changes:

• गोली → गोलियाँ for plural
• है → हैं for plural

And लेनी stays feminine, because गोलियाँ is feminine plural.