sudosereul naeneun nareul kkamppakhaeseo aebe memohae dwosseo.

수도세 means water tax / water utility bill / water charges. In everyday usage, it usually refers to the money you have to pay for water service.

• 수도 = water supply, running water
• 세 = tax/charge/fee

Depending on context, English translations like water bill or water charges often sound more natural than the literal water tax.

수도세를 내는 날 literally means the day on which I pay the water bill.

This uses a relative clause:

• 수도세를 내는 = paying the water bill
• 날 = day

So the whole chunk means the day for paying the water bill or the day I have to pay the water bill.

Korean very often modifies nouns this way instead of using structures like the day when....

The -는 is the present/adnominal form used to modify a noun.

• 내다 = to pay
• 내는 = paying / that one pays
• 내는 날 = the day one pays

Even though English might say the day to pay the bill or the payment day, Korean naturally uses verb + 는 + noun.

Here it does not necessarily mean present tense in the English sense. It is just the normal way to describe a noun with an action.

Because 깜빡하다 here takes 날 as its object: to forget the day.

So:

• 수도세를 내는 날을 = the day to pay the water bill
• 깜빡해서 = because I forgot / since I blanked on it

In English, you might think of the whole idea as I forgot the day for paying the water bill.

Here, 깜빡하다 means to forget momentarily, to blank on, or to let something slip your mind.

It often has a slightly casual, everyday feeling. It can suggest that the forgetting was:

• brief
• absent-minded
• unintentional

So 날을 깜빡해서 sounds like:

• because I forgot the date/day
• because it slipped my mind
• because I blanked on the day

Both can mean to forget, but the nuance is a little different.

• 깜빡하다: to forget absent-mindedly, to blank on something for a moment
• 잊어버리다: to forget, often with a stronger sense that something is fully forgotten or left undone

In this sentence, 깜빡해서 feels very natural because forgetting a due date is often an absent-minded mistake.

Compare:

• 날을 깜빡했어 = I totally spaced on the date.
• 날을 잊어버렸어 = I forgot the date.

Both work, but 깜빡했어 feels more colloquial and vivid.

The -아서/어서 form often connects two actions, and one common meaning is because / so.

So:

• 깜빡해서 = because I forgot / so I forgot and...
• 앱에 메모해 뒀어 = I made a note in the app

Together, the logic is:

Because I forgot the day to pay the water bill, I wrote it down in the app.

In context, the cause-and-result meaning is very natural.

메모해 뒀어 is short for 메모해 두었어.

The pattern V-아/어 두다 means:

• do something now
• and leave it that way for later
• often for convenience, preparation, or future use

So:

• 메모했어 = I wrote a note.
• 메모해 뒀어 = I wrote a note down and left it there for later / I made a note of it in advance.

That extra 두다 gives a sense of preparation or keeping something ready.

Yes.

• 해 두었어 → 해 뒀어

This kind of contraction is very common in spoken Korean.

Other similar examples:

• 써 두었어 → 써 뒀어
• 넣어 두었어 → 넣어 뒀어
• 예약해 두었어 → 예약해 뒀어

So 메모해 뒀어 is a very natural conversational form.

Because 에 marks the place/location where the memo is put.

• 앱에 메모하다 = to make a note in an app

Here the app is treated like the location or container where the memo exists.

If you used 으로, it would suggest by means of / using the app, which is a different nuance. That might be possible in some contexts, but 앱에 메모해 뒀어 is the most natural way to say I made a note in the app.

The subject is omitted, which is very common in Korean.

From context, the implied subject is I:

(나는) 수도세를 내는 날을 깜빡해서 앱에 메모해 뒀어.

Korean often leaves out subjects when they are obvious from context. In natural conversation, including 나는 every time would often sound unnecessary.

It literally means day, but in this kind of context it can naturally refer to the due date or the day/date when something must be paid.

So in English, depending on context, you might translate it as:

• the day
• the due date
• the payment date

That is why the full sentence may sound more natural in English as something like I kept forgetting the water bill due date, so I made a note in the app.

뒀어 is casual/informal polite-neutral speech? Actually, more precisely, it is informal, non-polite speech.

The ending -어 is used with:

• friends
• family
• people younger than you
• casual self-talk

So 메모해 뒀어 is casual: I made a note of it.

If you wanted a polite version, you could say:

수도세를 내는 날을 깜빡해서 앱에 메모해 뒀어요.

Not exactly.

The sentence says:

수도세를 내는 날을 깜빡해서
= because I forgot the day/date to pay the water bill

So the thing forgotten is the day, not necessarily the act of paying itself.

If you wanted to say I forgot to pay the water bill, Korean would more directly be something like:

수도세 내는 걸 깜빡했어.
or
수도세를 내는 걸 잊어버렸어.

That is a different meaning.

It is very natural conversational Korean.

A native speaker would easily understand it as:

• I kept forgetting the water bill due date, so I noted it in the app.
• I forgot the day to pay the water bill, so I made a note in the app.

The casual tone comes from:

• 깜빡해서
• 메모해 뒀어
• omission of the subject

All of that makes it sound like ordinary spoken Korean.