hwabuni neomu manheumyeon sikmure mul juneun de sigani orae geollyeo.

Because 많다 (to be many) treats the thing that is numerous as its subject, not its object.

So:

• 화분이 많다 = there are many flowerpots / potted plants
• not 화분을 많다

Here, -면 is added to 많다:

• 많으면 = if there are many / if it is many

So 화분이 너무 많으면 literally works like if flowerpots are too many.

화분 literally means flowerpot or plant pot. In everyday Korean, though, it can also refer to a potted plant by extension.

So in this sentence, 화분이 너무 많으면 can feel like:

• if there are too many pots
• or more naturally, if you have too many potted plants

That fits the second half, which talks about watering the plants.

Here 너무 means too.

• 많다 = to be many
• 많으면 = if there are many
• 너무 많으면 = if there are too many

Although 너무 can sometimes be used casually like very, in a sentence with a negative consequence like this, it is best understood as too:

• If there are too many, it takes a long time...

-면 means if or sometimes when.

So:

• 많다 → 많으면
• if it is many / if there are many

In this sentence, it introduces the condition:

• 화분이 너무 많으면 = if there are too many potted plants

It is a very common conditional ending in Korean.

Korean uses 물(을) 주다 literally to give water, and the plant is the target that receives the water.

So the normal pattern is:

• 식물에 물을 주다 = to water the plants
• literally: to give water to the plants

That is why 식물에 is used, not 식물을.

A few notes:

• 식물에 = to the plants
• 물(을) 주다 = give water
• Together, this means water the plants

In conversational Korean, the 을 on 물 is often omitted, so 물 주다 is very common instead of the fuller 물을 주다.

물 주는 데 uses the bound noun 데, which here means something like:

• in doing...
• for doing...
• when it comes to doing...

So:

• 물 주다 = to water
• 물 주는 데 = in watering / for watering

Then the rest of the sentence says time is required:

• 식물에 물 주는 데 시간이 오래 걸려
• It takes a long time to water the plants

This is a very common pattern:

• V-는 데 시간이 걸리다 = it takes time to do V

Examples:

• 집에 가는 데 한 시간 걸려요. = It takes one hour to get home.
• 책 읽는 데 오래 걸렸어요. = It took a long time to read the book.

Because this 데 is a bound noun, not a verb ending.

So it is spaced like a noun:

• 물 주는 데
• not 물주는데 in this meaning

Important distinction:

• -는데 as a connective ending is attached to the verb
  
  • example: 가는데 비가 와요 = I’m going / when I go, it rains
• 데 as a noun meaning place / situation / in doing is written separately
  
  • example: 가는 데 오래 걸려요 = It takes a long time to go

In your sentence, it is the second one: 주는 데.

The expression 시간이 걸리다 means time is required / it takes time.

So:

• 시간이 걸리다 = to take time
• 시간이 오래 걸리다 = to take a long time

Here:

• 시간이 = time (subject)
• 오래 = for a long time
• 걸려 = takes

This may feel different from English, where we often say it takes a long time.
In Korean, the structure is more like:

• time takes long / time is required for long

So the full sentence is built around:

• 식물에 물 주는 데 = for watering the plants
• 시간이 오래 걸려 = a long time is required

오래 is an adverb meaning for a long time.

So:

• 오래 걸리다 = to take a long time

This is the natural expression. Korean usually says:

• 시간이 오래 걸리다

rather than using something like 긴 시간이 걸리다 in this kind of everyday sentence.

Compare:

• 오래 걸려요 = it takes a long time
• 많이 걸려요 would mean it takes a lot, but that is less natural for time in this exact pattern unless the context is clearer

So 오래 걸려 is the standard choice.

걸려 is the plain informal style, often used in conversation, writing, or examples.

Compare:

• 걸려 = casual/plain
• 걸려요 = polite
• 걸립니다 = formal

So the same sentence could also be said as:

• 화분이 너무 많으면 식물에 물 주는 데 시간이 오래 걸려요.

That would sound more polite and is probably what a learner would use in many real-life situations.

They are similar, but 식물에 is the more natural choice here.

• 에 is commonly used with things like plants when talking about applying or giving something to them
• 에게/한테 is more strongly associated with animate recipients such as people or animals

So:

• 식물에 물을 주다 = natural
• 식물에게 물을 주다 = understandable, but less typical

For plants, 에 is the usual particle.

They are related, but not identical:

• 화분 = flowerpot / potted plant
• 식물 = plant

The sentence starts by talking about having many potted plants or pots, then refers to the action of watering the plants in them.

So the logic is:

• If there are too many pots/potted plants,
• watering the plants takes a long time.

Using both words is natural because the first part is about quantity of the items you own, and the second part is about the actual living things you water.

Yes. 물을 주는 데 is the fuller form, and 물 주는 데 is a very common shortened conversational form.

So both are possible:

• 식물에 물을 주는 데 시간이 오래 걸려요.
• 식물에 물 주는 데 시간이 오래 걸려요.

The version without 을 is especially common in speech and casual writing.

It breaks down like this:

• 화분이 너무 많으면
  = if there are too many potted plants
• 식물에 물 주는 데
  = in watering the plants / to water the plants
• 시간이 오래 걸려
  = it takes a long time

So the overall pattern is:

• [condition] + [action]-는 데 + 시간이 오래 걸리다

A very literal version would be:

• If potted plants are too many, in giving water to the plants, time takes long.

A natural English version is:

• If there are too many potted plants, it takes a long time to water them.