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

は marks 今日 as the topic: as for today / today.

So 今日は does not just mean today in a neutral sense; it sets the time frame for the sentence. A very natural literal feel is:

As for today, at the supermarket, eggs were on special, so I bought a lot.

Japanese often marks time expressions with は when they are being treated as the topic or contrastive setting.

で marks the place where something happens.

So スーパーで means at the supermarket or in the supermarket.

Here, the event/state being talked about is that the eggs were on special there. Japanese commonly uses で for the location of an event, action, or situation like this.

• スーパーで買いました = bought them at the supermarket
• スーパーで特売だった = they were on special at the supermarket

Because 特売だった describes the eggs; it does not take a direct object.

卵が特売だった means eggs were on sale / eggs were a special item.

Here, 卵 is the thing being identified as having the property of being on special, so が is natural.

You use を with verbs that take a direct object, such as:

• 卵を買いました = bought eggs

But 特売だった is not that kind of verb phrase. It is more like:

• X が Y だった = X was Y

So:

• 卵が特売だった = eggs were a special-sale item

特売 means special sale, sale item, or being sold at a special price.

In this sentence, 卵が特売だった means the eggs were being sold cheaply as part of a sale.

A natural English translation is:

• Eggs were on special
• Eggs were on sale

Even though 特売 is a noun, Japanese often uses noun + だ / だった to say something is/was that thing.

This is a very common pattern in Japanese.

When a clause comes before ので, から, けど, が, and similar connectors, it often uses the plain form, even in an overall polite sentence.

So this is completely natural:

• 卵が特売だったので、たくさん買いました。

The final verb of the sentence, 買いました, carries the politeness for the whole sentence.

A fully polite version like 特売でしたので is possible, but it sounds more formal/stiff. The mixed style here is standard everyday Japanese.

Both can mean because, but the nuance is different.

• ので sounds a bit softer, more explanatory, and less direct
• から can sound more straightforward, assertive, or conversational

So:

• 卵が特売だったので、たくさん買いました。
• 卵が特売だったから、たくさん買いました。

Both mean roughly Because eggs were on sale, I bought a lot, but ので feels slightly gentler and more neutral.

たくさん means a lot or many.

Here it modifies 買いました, so たくさん買いました means:

• bought a lot
• bought many (of them)

The thing bought is understood to be the eggs mentioned earlier.

Japanese often leaves repeated objects unstated when they are obvious from context. So this is essentially:

• (卵を) たくさん買いました = I bought a lot (of eggs)

It is omitted, which is very normal in Japanese.

From context, 買いました implies that the speaker is the one who bought them, so in English we translate it as I bought a lot.

Japanese often leaves out:

• the subject
• the object
• anything else that is clear from context

So even though I is not said, it is naturally understood.

A helpful breakdown is:

• 今日は = as for today
• スーパーで = at the supermarket
• 卵が特売だったので = because eggs were on sale
• たくさん買いました = I bought a lot

So the overall structure is:

[time/topic] + [place] + [reason] + [main action]

Japanese often puts the reason before the main result/action.

Yes, that is the idea, but Japanese is slightly less explicit.

The sentence literally says something closer to:

Because eggs were on special, I bought a lot.

The word 卵 is not repeated after 買いました, because it is already understood. In English, we often make it explicit:

• I bought a lot of eggs

Japanese does not need to repeat it.

The main readings are:

• 今日 → きょう
• スーパー → スーパー
• 卵 → たまご
• 特売 → とくばい
• 買いました → かいました

So the full sentence is read:

きょう は スーパー で たまご が とくばい だった ので、たくさん かいました。

Yes, it could, but the nuance changes.

• 卵が特売だった focuses on eggs as the thing that was on sale
• 卵は特売だった gives eggs more of a topic or contrastive feel, like as for eggs, they were on sale

In this sentence, が sounds very natural because it presents the information straightforwardly: the eggs were the sale item.

Using は might suggest contrast, for example:

• 卵は特売だったけど、牛乳は高かった。
  • Eggs were on sale, but milk was expensive.

So が is the more neutral choice here.