rezi de ryousyuusyo wo morauno wo wasureta ga, tenin ni ittara sugu dasite kureta.

で marks the place where an action happens.

So レジで means at the register / at the checkout.
It tells you where the action of receiving the receipt was supposed to happen.

• レジで領収書をもらう = to get a receipt at the register

If it were レジに, that would sound more like direction or destination, not the location of the action.

領収書 is a receipt, but specifically a more formal receipt, often used for business expenses, reimbursement, tax records, etc.

レシート is the ordinary store receipt you usually get automatically.

So:

• レシート = everyday till receipt
• 領収書 = formal receipt, often requested

In many stores in Japan, you may need to ask for a 領収書.

Here, の turns the verb phrase 領収書をもらう into a noun-like unit: getting a receipt.

Then that whole action becomes the thing that was forgotten.

So:

• 領収書をもらう = to receive a receipt
• 領収書をもらうの = receiving a receipt / getting a receipt
• 領収書をもらうのを忘れた = forgot to get a receipt

This is a very common pattern:

• V-dictionary form + のを忘れる = to forget to do V

Examples:

• 買うのを忘れた = I forgot to buy it.
• 持ってくるのを忘れた = I forgot to bring it.

That を marks the noun-like phrase もらうの 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 [getting the receipt].

It may look strange because there is already an を inside the phrase:

• 領収書をもらう → the receipt is the object of もらう
• もらうのを忘れた → the whole act of receiving it is the object of 忘れた

So the two を are doing different jobs.

Yes, 領収書をもらうことを忘れた is grammatically possible.

But の often sounds more natural for a concrete, specific action, especially in everyday speech.

Compare:

• 領収書をもらうのを忘れた = very natural in conversation
• 領収書をもらうことを忘れた = also correct, a little more formal or abstract

In sentences about forgetting a specific action, のを忘れた is extremely common.

Yes, here が means but or however.

It connects two ideas:

1. I forgot to get the receipt at the register
2. but when I told the store clerk, they immediately issued one for me

In written or slightly formal spoken Japanese, が is a common way to connect contrasting statements.

You could also say けど in more casual speech:

• 忘れたけど、店員に言ったら…

So が here is basically a neutral but.

Because 言う uses に for the person you say something to.

• 店員に言う = to tell the clerk
• 友達に言う = to tell a friend

So に marks the target/listener of the speech.

The thing you say can be marked in different ways, often omitted if obvious.

Here, what was said is not stated explicitly, because it is understood from context: the speaker told the clerk that they had forgotten the receipt / wanted a receipt.

Here 〜たら means when or if.

• 店員に言ったら、すぐ出してくれた
• When I told the clerk, they immediately issued one for me.

In this sentence, it refers to what happened after that action.

A helpful way to understand it:

• Xたら、Y = when/if X happened, Y happened

Because the result actually occurred, when is a very natural translation here.

すぐ means immediately / right away and modifies the verb phrase that follows.

• すぐ出してくれた = issued it right away

Its position is very normal. Japanese adverbs often come right before the verb or verb phrase they modify.

You could think of it as:

• 店員に言ったら、[すぐ][出してくれた]

In this context, 出す means to issue / provide / bring out.

For things like receipts, documents, invoices, certificates, etc., Japanese often uses 出す.

So:

• 領収書を出す = to issue a receipt

Then くれた adds the nuance that the action was done for the speaker’s benefit:

• 出してくれた = (they) issued it for me

So the feeling is not just they issued it, but they kindly did that for me.

くれる is used when someone does something for the speaker or someone close to the speaker.

So:

• 出した = they issued it
• 出してくれた = they issued it for me

This gives the sentence a personal, beneficiary nuance. It often suggests appreciation, helpfulness, or that the action was favorable to the speaker.

That is why 出してくれた feels more natural than just 出した in this situation.

Japanese often leaves the subject unstated when it is clear from context.

In this sentence:

• (I) forgot to get the receipt at the register
• but when (I) told the clerk, (the clerk) immediately issued one for me

So the subject changes, but Japanese does not need to state that explicitly.

You infer it from the verbs:

• 忘れた → usually the speaker, in this kind of sentence
• 言ったら → also the speaker
• 出してくれた → the clerk did the action, because くれた means someone did something for the speaker

This kind of subject omission is extremely common in Japanese.

Because 店員 here means the store clerk / a store employee as a role, not necessarily as a polite way of directly addressing them.

The sentence is describing what happened, not calling out to the clerk.

So:

• 店員に言ったら = when I told the store clerk
• not when I told Mr./Ms. Clerk

If you were speaking to the clerk directly, you would usually not say 店員さん to their face in this way either; you would more likely just speak politely without using that label, or use something like すみません to get their attention.

This sentence is in plain style:

• 忘れた
• 言ったら
• 出してくれた

That is normal in casual speech, diary style, storytelling, and many textbook example sentences.

A polite version would be something like:

• 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。

Both are correct. The difference is mainly tone:

• plain form = casual / neutral narrative
• polite form = more formal / polite

No, in normal usage it means I forgot to receive/get the receipt at the register.

The pattern V-dictionary form + のを忘れる usually means forgot to do V, not forgot that V happened.

If you wanted to say I forgot that I got the receipt, you would usually say it differently, for example with a past verb phrase:

• 領収書をもらったことを忘れた

But even that would depend on context. In your sentence, the intended meaning is clearly I forgot to get it.

Yes. This sentence uses several very common everyday patterns:

• Vるのを忘れた = forgot to do V
• Nに言ったら = when I told N
• すぐ Vてくれた = someone did something right away for me

So this is a very useful model sentence. You can swap in other words:

• 駅で切符を買うのを忘れたが、係員に言ったらすぐ対応してくれた。
• 宿題を出すのを忘れたが、先生に言ったら受け取ってくれた。

It is a very practical structure for describing a small problem and how someone helped fix it.