Learn Hardcore Japanese: rezi de ryousyuusyo wo morauno wo wasureta ga, tenin ni ittara sugu dasite kureta. - I forgot to get a receipt at the register, but when I told the clerk, they issued it right away.

The particle で marks the place where an action happens, so レジで means “at the register.” Use に when you’re talking about a destination or target, as in movement or pointing toward something. For example, レジに並ぶ (line up at the register) or レジに行く (go to the register). For doing something at that location, で is correct: レジで払う／レジで領収書をもらう.

What does the の in 領収書をもらうのを忘れた do? Why are there two を?

の nominalizes the verb phrase 領収書をもらう, turning “get a receipt” into a noun-like chunk so it can be the object of 忘れる. The first を marks the object of もらう (the receipt), and the second を marks the whole nominalized action as the object of 忘れた. So the structure is: [領収書をもらう]のを忘れた = “I forgot to get a receipt.”

Can I use こと instead of の here?

Yes: レジで領収書をもらうことを忘れた is grammatically fine. の feels more conversational and immediate; こと sounds a bit more formal or abstract. With 忘れる, both are acceptable, but many speakers prefer 〜のを忘れた in everyday speech.

Is there a shorter or more natural way to say “forgot to get a receipt”?

A common compact pattern is the compound verb もらい忘れる: レジで領収書をもらい忘れた. If you want to add a nuance of regret or “oops,” use 〜てしまう: レジで領収書をもらい忘れてしまった (casual: もらい忘れちゃった).

What does が do here? Is it the subject marker?

Here が is a conjunction meaning “but/although,” linking two clauses: 忘れたが、…. It’s not the subject marker. You could also use けど/けれど(も); けど is more casual, が is a bit neater or written-like. Starting a new sentence with でも is another option for a looser connection.

Why is it 言ったら? How is that different from 言えば, 言うと, or 言うなら?

〜たら with a past verb often means “when/after I did X (then Y happened),” marking a sequence in a specific instance: 言ったら、すぐ出してくれた.

言えば is a hypothetical/generic “if”: 言えば、出してくれる (in general, if you ask, they’ll give it).
言うと tends to express a direct, regular reaction: 言うと、すぐ出してくれる (whenever you say it, they promptly issue it).
言うなら frames a condition or topic (“if it’s a matter of saying/if you say so”), which doesn’t fit as naturally here.

Who is the subject of 出してくれた? It isn’t stated.

It’s understood to be the clerk: （店員が）出してくれた. With くれる, the subject is the giver/doer who does a favor for the speaker, and the beneficiary (me/us) is implicit. The object （領収書を） is also understood from context.

Could I say 出してもらった instead of 出してくれた? What’s the difference?

Yes: （店員に）出してもらった is also natural.

〜てくれる highlights the doer’s kindness from the receiver’s perspective (“they kindly did X for me”).
〜てもらう highlights that the speaker received the favor (“I got them to do X / I had X done for me”).
Meaning is effectively the same here; it’s a vantage-point preference.

Why is 店員 marked with に? Does it belong to 出してくれた?

The に goes with 言った: 店員に言った = “told the clerk.” In 出してくれた, the subject (the clerk) is implicit and would normally be 店員が if stated. If you switch to もらう, then 店員に would be the giver: 店員に出してもらった.

Is 出す the right verb for receipts? Are there alternatives?

Yes, 領収書を出す is everyday, natural Japanese for “issue/hand out a receipt.” Alternatives: 発行する (more formal, “issue”), 渡す (hand over), and in shop talk, 領収書を切る (“write up a receipt”). In this context, 出してくれた sounds perfectly natural.

What’s the difference between 領収書 and レシート?

領収書 is a formal receipt, often bearing a name/stamp, used for accounting or reimbursement. レシート is the regular till receipt you automatically get. Some companies accept a レシート as proof, but when they specifically ask for a formal receipt, that’s 領収書.

Could I say レジで領収書を忘れた? Would that mean the same thing?

No. レジで領収書を忘れた means “I forgot the receipt (left it/forgot to take it) at the register.” To say you forgot to obtain one, use 領収書をもらうのを忘れた or 領収書をもらい忘れた.

How would I make the sentence more polite or more humble?

Polite but neutral: レジで領収書をもらうのを忘れましたが、店員に言ったらすぐ出してくれました。
More humbly (about yourself) or more respectful toward the clerk: レジで領収書をもらい忘れてしまいましたが、店員さんにお願いしたら、すぐ出していただきました。
Using 〜ていただく is the humble counterpart of receiving a favor.

What does すぐ modify? Is すぐに any different?

すぐ is an adverb modifying 出してくれた: “they issued it immediately.” すぐに is essentially the same; に is optional. Both are natural: すぐ出してくれた / すぐに出してくれた.

What exactly did I “say to the clerk”? What are natural phrases to use?

The content is implied (e.g., that you forgot). Natural ways to ask are: 領収書をお願いします。, 領収書をいただけますか。, or 領収書を発行していただけますか。 If clarifying the situation: すみません、領収書をもらい忘れました。いただけますか。