jinandareneun gaseubiga saenggakboda jeokge nawaseo dahaengieosseo.

-에는 is 에 + 는.

• 에 marks the time: in/at
• 는 makes 지난달 the topic and often adds a slight contrast

So 지난달에는 feels like:

• as for last month
• last month, at least
• when it comes to last month

This can imply a comparison with other months, even if it is not stated directly.

• 지난달에 가스비가 적게 나왔어 = The gas bill was low last month.
• 지난달에는 가스비가 적게 나왔어 = As for last month, the gas bill was low.
  (Maybe other months were different.)

Because in this sentence, 가스비 is the subject of 나오다.

Korean often uses 나오다 for things like:

• bills
• prices
• scores
• results

So the literal structure is close to:

• The gas bill came out low
• The amount of the gas bill came out lower than expected

That is why 가스비가 is natural here.

If you said 가스비는, you would be making gas bill the topic instead:

• 가스비는 생각보다 적게 나왔어
  = As for the gas bill, it came out lower than expected.

Both are possible, but 가스비가 fits the basic subject role very naturally.

Yes, but here it is being used in an extended, very common Korean way.

With bills, prices, scores, and test results, 나오다 often means:

• to come out
• to turn out
• to be charged
• to be shown as

So 가스비가 적게 나왔다 does not mean the gas bill physically came out somewhere. It means:

• the gas bill amount turned out low
• the billed amount was small
• the charge came out lower than expected

Similar examples:

• 시험 점수가 잘 나왔어. = My test score came out well.
• 수리비가 많이 나왔어. = The repair cost came out high.
• 생각보다 가격이 싸게 나왔어. = The price came out cheaper than expected.

Because 적게 is the adverb form, and it modifies 나오다.

• 적다 = to be few / to be small in amount
• 적은 = adjective form used before a noun
• 적게 = adverb form, meaning in a small amount / low

Here the idea is:

• the gas bill came out low
• not the gas bill is a small bill

So:

• 적게 나오다 = to come out low / to be low in amount

Compare:

• 적은 돈 = a small amount of money
• 돈이 적게 나왔다 = the amount of money came out low

생각보다 means than I thought or than expected.

It is made from:

• 생각 = thought
• 보다 = than / compared with

So 생각보다 적게 means:

• lower than I thought
• less than expected

This expression is very common and useful. You can use it with many things:

• 생각보다 맛있어. = It’s tastier than I thought.
• 생각보다 어려웠어. = It was harder than I expected.
• 생각보다 사람이 많아. = There are more people than I thought.

It does not always literally mean you actively sat and thought about it. Often it just means than expected.

Here -아서 / -어서 connects two clauses and shows the reason.

So:

• 적게 나와서 = because it came out low / since it came out low

The full logic is:

• The gas bill came out lower than expected,
• so it was a relief.

In this sentence, -서 is functioning like because.

Very similar examples:

• 비가 와서 못 갔어. = I couldn’t go because it rained.
• 시험이 끝나서 좋아. = I’m happy because the exam is over.
• 가격이 싸서 샀어. = I bought it because it was cheap.

Because the speaker is talking about a past situation and evaluating it as fortunate.

• 다행이야 = It’s fortunate / That’s a relief.
• 다행이었어 = It was fortunate / It was a relief.

Since the sentence begins with 지난달에는, the whole situation is placed in the past, so 다행이었어 sounds very natural.

In English, we might still say That was a relief when looking back on something. Korean does the same.

Not exactly.

다행이다 is often closer to:

• it’s a relief
• that’s fortunate
• I’m glad that worked out
• thank goodness

It does not always mean luck in the random-chance sense.

In this sentence, the feeling is more like:

• I was relieved because the gas bill was lower than expected.

So 다행이었어 carries emotional relief, not just abstract luck.

The sentence ends in -었어, which is casual, plain speech.

• 다행이었어 = casual
• 다행이었어요 = polite

So the polite version would be:

지난달에는 가스비가 생각보다 적게 나와서 다행이었어요.

Everything else can stay the same.

Yes. Korean often leaves out subjects when they are obvious from context.

This sentence does not explicitly say:

• I was relieved
• It was fortunate for me

But that feeling is understood from 다행이었어.

So the speaker’s point of view is implied naturally. Korean does this all the time, especially with feelings, opinions, and reactions.

Yes, that is also natural.

Compare the nuance:

• 지난달에는 가스비가...
  = As for last month, the gas bill...
• 지난달 가스비는...
  = As for last month’s gas bill...

The first version highlights last month as the topic/time frame.
The second highlights last month’s gas bill as the topic.

Both are correct; they just focus the sentence slightly differently.

It can mean either, depending on context.

• 가스비 = gas expense / gas bill / gas charge

In a sentence with 나오다, it often strongly suggests the billed amount or charge:

• 가스비가 적게 나왔다 = the gas bill came out low

In other contexts, it can simply mean money spent on gas.

This is common with -비, which means expense/cost:

• 식비 = food expenses
• 교통비 = transportation costs
• 관리비 = maintenance fee / building fee