Learn Hardcore Japanese: kinou ha keeki wo hutatu tabetakoto wo sukosi koukaisite imasu. - I somewhat regret having eaten two pieces of cake yesterday.

昨日 is a time word. In Japanese, very common time expressions like 昨日, 今日, 明日, 毎日 usually appear without に when they simply say “when” something happened:

昨日ケーキを食べました。 – I ate cake yesterday.
明日映画を見ます。 – I’ll watch a movie tomorrow.

In this sentence, 昨日は makes 昨日 the topic of the sentence:

昨日はケーキを二つ食べたことを少し後悔しています。
→ As for yesterday / Speaking about yesterday, I somewhat regret having eaten two pieces of cake.

So は is marking 昨日 as “what we’re talking about,” not as a time-marking particle.
昨日に would be grammatical in some other contexts, but for the simple “yesterday I did X,” you normally say 昨日Xした, not 昨日にXした.

Why is it ケーキを二つ instead of 二つのケーキ?

Both are possible, but they sound different.

ケーキを二つ食べた。
→ Literally “ate cake, two (pieces).”
This is the most natural, everyday way to say “I ate two pieces of cake.”
二つのケーキを食べた。
→ Literally “ate two cakes (two individual cakes).”
This sounds more like “two separate whole cakes” as individual items, and also has a slightly more formal or “bookish” feel in many contexts.

With food portions, native speakers usually prefer:

[thing] を [counter] [verb]
ケーキを二つ食べる, 水を一杯飲む, りんごを三個買う, etc.

So ケーキを二つ食べた is the natural choice for “ate two pieces of cake.”

Why is there no particle after 二つ? Why not 二つを食べた?

The structure is:

ケーキ (object)
を (object marker)
二つ (counter)
食べた (verb)

In Japanese, counters like 二つ almost always come right before the verb, without a particle:

本を三冊買いました。 – I bought three books.
水を一杯飲みました。 – I drank one glass of water.
ケーキを二つ食べました。 – I ate two pieces of cake.

You can say things like 二つを食べた, but in that case 二つ is being treated as a standalone noun (“the two (of them)”), not just as a counter attached to ケーキ. It would have a different nuance, like “I ate the two (ones),” with the cakes already understood from context.

Here, we just want “two cakes,” so ケーキを二つ食べた is the normal pattern.

What does 食べたこと literally mean, and why use こと here?

食べたこと is:

食べた – the plain past form of 食べる (to eat)
こと – a noun meaning “thing / fact / occurrence” used to nominalize verbs

So 食べたこと literally means “the fact of having eaten” or “the event of eating.”

We need こと because 後悔する (“to regret”) takes a noun as its object.
We can’t directly make a verb phrase into the object, so we turn it into a noun phrase:

そのことを後悔しています。 – I regret that thing.
ケーキを二つ食べたことを後悔しています。
→ I regret the fact that I ate two pieces of cake.

This pattern is very common:

日本に行ったことがあります。 – I have been to Japan (I have the experience of having gone).
彼にひどいことを言ったことを後悔している。
– I regret having said something awful to him.

So V-た + こと = “the fact that (I) did V.”

Why is it 食べたこと (past) and not 食べること?

食べたこと refers to something that actually happened in the past: the fact that you ate (two cakes).

If you said 食べることを後悔している, it would sound like you regret eating cake in general or “the act of eating cake (as a general habit).” That doesn’t fit the specific “yesterday I ate two cakes” situation.

So:

ケーキを二つ食べたことを後悔している。
→ I regret that I ate two pieces of cake (a specific event in the past).
ケーキを食べることを後悔している。
→ I regret eating cake (as a general behavior/habit), or regret the idea of eating cake.

Here, the past form 食べたこと is required to point to that particular action yesterday.

Aren’t you only allowed one を in a sentence? Here I see ケーキを and 食べたことを.

The “only one を per sentence” rule is a simplification that’s often taught at the very beginning.

What actually matters is that one clause doesn’t have two direct objects with を. But a complex sentence can have:

An inner clause with its own を
An outer clause with its own を

In your sentence:

Inner clause: ケーキを二つ食べた
- Verb: 食べた
- Direct object: ケーキを
Outer clause: [ケーキを二つ食べたこと] を後悔している
- Verb: 後悔している
- Direct object: 食べたことを (the whole inner clause turned into a noun with こと)

So the structure is:

[ケーキを二つ食べたこと] を後悔している。

Because there are two clauses, it is perfectly fine to have two を’s. Each を belongs to a different verb.

What’s the nuance of 少し here? Could you use ちょっと instead?

少し means “a little / a bit” in a fairly neutral or slightly formal way.

少し後悔しています。
→ I regret it a little.

You could say:

昨日はケーキを二つ食べたことをちょっと後悔しています。

ちょっと is very common in speech, a bit more casual and colloquial. In many everyday contexts 少し and ちょっと are interchangeable, but:

少し tends to feel a bit more neutral/polite.
ちょっと often sounds more spoken and can carry more emotional nuance (“kind of / sort of / a little bit”).

In this sentence, 少し makes the tone mildly polite and neutral, which matches 〜しています nicely.

Why is it 後悔しています and not just 後悔します?

後悔している is the ている form of 後悔する.

後悔する – to regret
後悔している – to be regretting / to be in a state of regret

The 〜ている form often expresses a current state rather than an action:

知っている – (I) know (state of having knowledge)
結婚している – (I am) married (current state)
疲れている – (I am) tired (state)

So 後悔している is “am in a state of regret,” which corresponds very well to the English “I regret (it)” or “I’m regretting it.”

Using 後悔します would sound more like a simple present/future act (“I will regret / I do regret” as a one-time act) and is less natural for this kind of ongoing feeling. 後悔しています is the normal, natural choice.

Why is the main verb polite (います) but 食べた is in plain form? Shouldn’t they match?

In Japanese, subordinate clauses (like relative clauses or embedded clauses) usually use the plain form, even when the main sentence is in the polite form.

Here:

Main predicate: 後悔しています – polite (です／ます style)
Embedded clause: ケーキを二つ食べたこと – plain past

This is completely standard:

昨日見た映画は面白かったです。
→ The movie I saw yesterday was interesting.
- 見た (plain) modifies 映画
- 面白かったです is polite
日本に行ったことがあります。
→ I have been to Japan.
- 行った (plain) in the embedded clause
- Whole sentence is fine in polite speech

So 食べた is in the plain past because it’s part of the embedded Vたこと, not the main sentence ending.

What exactly does 後悔しています mean in terms of time? Is it “I regret” or “I am regretting”?

Japanese 〜ている doesn’t map perfectly to English “-ing,” but here the nuance is:

A current, ongoing mental state: “I (now) feel regret about that.”

Depending on context, you could translate 後悔しています as:

“I regret (it).”
“I’m regretting it.”
“I have regrets about it.”

The important idea is that this regret is present and continuing, not something that just flashed by in the past. English would usually just say “I regret eating two pieces of cake yesterday,” and that matches 後悔しています well.

Why is there no I (like 私は) in the Japanese sentence?

Japanese often omits the subject when it’s clear from context. In this sentence, it’s natural to assume the speaker is talking about themselves.

So:

昨日はケーキを二つ食べたことを少し後悔しています。

is understood as:

（私は）昨日はケーキを二つ食べたことを少し後悔しています。
→ (I) somewhat regret having eaten two pieces of cake yesterday.

Adding 私は is not wrong, but it’s often unnecessary and can sometimes even feel a bit heavy or contrastive (like “As for me, I regret…”). In normal conversation, leaving it out is more natural.

Is there any difference between this sentence and:
昨日はケーキを二つ食べて、少し後悔しています。

Both are natural and very close in meaning, but the structure is slightly different.

昨日はケーキを二つ食べたことを少し後悔しています。
- Uses 食べたことを後悔しています
- Focuses on “I regret the fact that I ate two pieces of cake.”
昨日はケーキを二つ食べて、少し後悔しています。
- Uses 〜て, linking two actions/states:
  - (I) ate two pieces of cake yesterday
  - (now) I regret it a little
- More like: “I ate two pieces of cake yesterday, and now I somewhat regret it.”

In everyday conversation, (2) with 〜て is very common and sounds a bit more casual/natural. (1) with 食べたことを後悔している is slightly more explicit and can feel a bit more “carefully worded” or formal, depending on context.

Is 二つ the usual counter for cake? Could you use another counter?

二つ is the generic counter for “two things,” and it’s perfectly natural here. For cake, other counters are also possible depending on nuance:

二つ – two pieces / two (items), generic and very common
二個 (にこ) – two individual pieces (slightly more concrete, often for small round things)
二切れ (ふたきれ) – two slices (emphasizes slice shape)
二ピース – two pieces (loanword, used for pizza, cake, etc., often on menus)

In everyday speech, if you just mean “two pieces of cake,” ケーキを二つ食べた is completely fine and widely used.