kyou ha suupaa de yooguruto ga tokubai datta node, takusan kaimasita.

は marks 今日 as the topic or time frame of the sentence: as for today.

It helps set the scene before the speaker says what happened. In English, we often just say Today, ... at the start of a sentence. Japanese often uses は for that kind of setup.

You could sometimes just say 今日 without は, especially in casual speech, but 今日は sounds more complete and natural here.

で marks the place where something happens.

Here, the supermarket is the place where the yogurt was on special sale and where the buying happened, so で is the natural particle.

に is more often used for:

• destinations
• existence
• points in time
• things you move toward

So スーパーに would not fit this sentence as well.

Because in the clause ヨーグルトが特売だった, ヨーグルト is the thing that was on sale.

So が marks the subject of 特売だった.

In other words, the sentence is not saying I special-saled yogurt or I did something to yogurt. It is saying yogurt was on special sale.

The object of 買いました is left unstated because it is already obvious from context: the speaker bought the yogurt.

Yes, but the nuance changes.

• ヨーグルトが特売だった = yogurt was the thing that was on sale
• ヨーグルトは特売だった = as for yogurt, it was on sale

が sounds more neutral and natural when introducing the item that had that condition.

は can sound more contrastive, as if comparing yogurt with other items:

• ヨーグルトは特売だったけど、牛乳は高かった
• Yogurt was on sale, but milk was expensive

So in this sentence, が is the more natural choice.

特売 is a noun meaning special sale, sale item, or special bargain.

In Japanese, nouns can be used as predicates. When a noun predicate is in the plain past, it takes だった.

So:

• 特売だ = is on special sale
• 特売だった = was on special sale

That is why it is 特売だった, not something like 特売かった. It is not an い-adjective.

Because the speaker is describing a situation that already happened earlier today.

The logic is:

• the yogurt was on sale
• because of that
• the speaker bought a lot

Even if everything happened on the same day, Japanese often uses past tense when recounting completed events or states that existed at the time of the action.

So 今日 does not force the sentence to be present tense.

Both ので and から can mean because or since, but they have a slightly different feel.

• ので: softer, more explanatory, more matter-of-fact
• から: more direct, sometimes more personal or assertive

In this sentence, ので sounds natural because the speaker is calmly explaining the reason for buying a lot.

Compare:

• ヨーグルトが特売だったので、たくさん買いました。
• ヨーグルトが特売だったから、たくさん買いました。

Both are correct, but ので sounds a little gentler and more explanatory.

That is completely normal in Japanese.

The politeness of the sentence is usually shown at the end of the main clause. Subordinate clauses before words like ので, から, けど, and とき are often in plain form, even in otherwise polite speech.

So this pattern is very common:

• 理由だったので、〜ました

A more formal version like 特売でしたので is also possible, but 特売だったので、買いました is standard and natural.

It is omitted.

Japanese very often leaves out subjects when they are clear from context. Since 買いました describes the speaker’s action, the listener naturally understands that the subject is I.

So the sentence does not need 私は.

If you added it, it would still be grammatical:

• 私は今日、スーパーでヨーグルトが特売だったので、たくさん買いました。

But in natural Japanese, leaving it out is very common.

Because Japanese often omits information that is already obvious.

The first clause already tells us that the yogurt was on sale, so when the speaker says たくさん買いました, the listener naturally understands bought a lot of yogurt.

Repeating ヨーグルト is possible, but it can sound a little repetitive:

• ヨーグルトが特売だったので、たくさんヨーグルトを買いました。

That is understandable, but the shorter version is more natural here.

Here たくさん works like an adverb and means a lot.

It modifies 買いました, so the meaning is bought a lot.

It does not need の in this structure.

Compare:

• たくさん買いました = bought a lot
• たくさんのヨーグルトを買いました = bought a lot of yogurt

So たくさん can act differently depending on what it modifies.

Yes.

That version makes the object explicit:

• 今日はスーパーでヨーグルトが特売だったので、たくさんのヨーグルトを買いました。

This is grammatically correct, but in everyday speech the original sentence sounds more natural because Japanese often avoids repeating nouns when the meaning is already clear.

So both are possible, but the original is more streamlined.

Not exactly.

• 安い means cheap or inexpensive
• 特売 means on special sale or being sold at a special promotional price

So:

• ヨーグルトが安かった = the yogurt was cheap
• ヨーグルトが特売だった = the yogurt was on special sale

Something can be 安い without being a 特売, and something can be described as a 特売 because it is part of a promotion.

Not completely, but the given order is very natural.

Japanese often places information before the verb, and this sentence flows in a common order:

• 今日 = time/topic
• スーパーで = place
• ヨーグルトが特売だったので = reason/background
• たくさん買いました = main action

Some parts can be moved for emphasis, but the original order is a very natural way to present the information.