nitiyouhin wo kai ni ittara, senzai no tumekae ga tokubai desita.

Because verb stem + に + 行く / 来る / 帰る is the standard pattern for going, coming, or returning to do something.

• 買う → verb stem 買い
• 買いに行く = go to buy

So:

• 日用品を買いに行く = go to buy daily necessities

Using 買うに行く would be unnatural.

Here, に marks purpose.

In 買いに行く, it shows the reason for going:

• 買いに = to buy
• 行く = go

So the pattern is:

• [verb stem] + に + movement verb

Examples:

• 食べに行く = go to eat
• 見に行く = go to see/watch
• 借りに行く = go to borrow

行ったら comes from 行く + たら.

In this sentence, it means something like:

• when I went
• after I went
• when I went, I found that...

A very natural nuance here is discovery: after going to buy household items, the speaker discovered that the detergent refills were on special.

So the sentence feels like:

• When I went to buy daily necessities, detergent refills were on sale.

Because たら often works well when one event happens and then the speaker notices or experiences another result.

Compare:

• 日用品を買いに行ったら、…
  = When I went to buy daily necessities, ...
  This often implies a result or discovery.

• 日用品を買いに行ったとき、…
  = When I went to buy daily necessities, ...
  This is more neutral, just locating something in time.

So たら here gives a nice “I went, and then it turned out that...” feeling.

日用品 means daily necessities or everyday household items.

It can include things like:

• soap
• toothpaste
• detergent
• tissues
• shampoo

Japanese nouns usually do not show singular/plural the way English does, so 日用品 can refer to one kind of daily-use item or several items, depending on context. In English, it is often translated as daily necessities or household essentials.

It means detergent refill or detergent refill pack.

Breakdown:

• 洗剤 = detergent
• 詰め替え = refill / replacement pack

So 洗剤の詰め替え is literally the refill of detergent, but in natural English we usually say:

• detergent refill
• refill pack of detergent

Because 詰め替え here is a noun, not a verb.

• 洗剤の詰め替え = detergent refill
  Here, の links two nouns.

If you used を, it would suggest 詰め替える as a verb:

• 洗剤を詰め替える = to refill/transfer detergent

That is a completely different structure and meaning.

So in this sentence, 詰め替え is talking about the product, not the action.

Because が marks the thing that had the property of being 特売.

So:

• 洗剤の詰め替えが特売でした = The detergent refills were on special

Here, が introduces the item that is being identified as the one on sale.

If you said:

• 洗剤の詰め替えは特売でした

that would sound more like “As for the detergent refills, they were on sale.”
That version is also possible, but が is very natural when presenting the information as something noticed or newly relevant.

Because 特売 is functioning as a predicate noun with the copula です / でした.

So:

• 特売でした = was on special sale / was a bargain item

This is similar to:

• 学生でした = was a student
• 雨でした = it was rain / it was rainy weather
• 休みでした = it was a day off

No particle is needed there.

Not exactly.

• 安い = cheap / inexpensive
• 特売 = special sale / special bargain / promotional price

So 安い describes price in general, while 特売 suggests a store promotion or special discount event.

For example:

• この洗剤は安いです。 = This detergent is cheap.
• この洗剤は特売です。 = This detergent is on special.

Because the speaker is talking about a completed past experience.

• 行ったら = when I went
• 特売でした = was on sale

The speaker is looking back and reporting what happened at that time.

If you changed the tense, the meaning would also shift. For example:

• 日用品を買いに行ったら、洗剤の詰め替えが特売です。

This would sound odd in most situations, because the first part is in the past but the second part is describing the sale as a present fact.

The subject is omitted because Japanese often leaves out information that is clear from context.

In English, you usually need to say I went.
In Japanese, if it is obvious that the speaker is talking about their own action, you can simply say:

• 日用品を買いに行ったら...

The understood subject is usually:

• I
• sometimes we, depending on context

This omission is very common and natural in Japanese.

In general, たら can express either:

• if
• when/after

But in this sentence, because the whole thing is describing a past event, 行ったら is understood as:

• when I went
• after I went

So here it is not really if I go, but rather when I went and then found that...