Learn Hardcore Japanese: kyou ha konbini no pan dake de hirugohan wo sumasemasita. - Today I just had some convenience store bread for lunch.

It doesn’t mean “today is” in the sense of a grammatical subject; it just frames what you’re going to talk about. The rest of the sentence is the comment about “today”:

今日は コンビニのパンだけで昼ご飯をすませました。
= As for today, I got by with only convenience‑store bread for lunch.

Why is there no word for “I” in the Japanese sentence?

Japanese often drops the subject when it’s obvious from context. In conversation, if you’re talking about your own day, it’s naturally understood that “I” is the one who ate lunch, so:

(私は) 今日はコンビニのパンだけで昼ご飯をすませました。

The 私は is simply omitted. Adding 私は is grammatically fine but often unnecessary and can sound a bit heavy if overused.

What does の do in コンビニのパン?

の links two nouns and usually shows a possessive or descriptive relationship. Here:

コンビニ = convenience store
パン = bread
コンビニのパン = bread of/from a convenience store

So コンビニのパン means “convenience‑store bread” or “bread from the convenience store.”

What does だけで mean here, and how is it different from just だけ?

だけ means “only / just.”
で (after a noun) can mark:

means / material / cause / condition, etc.

In コンビニのパンだけで, the combination だけで means:

“with only …”
“by just …”
“using nothing but …”

So:

コンビニのパンだけで昼ご飯をすませました。
= I managed lunch with only bread from the convenience store.

If you said just コンビニのパンだけ, it would mean “only convenience‑store bread,” but adding で clearly shows it is the means/material for dealing with lunch.

How is だけ different from しか in this kind of sentence?

Both relate to “only,” but they behave differently:

だけ
- Works with affirmative verbs.
- Neutral: just states that something is limited.
- Example:
  - コンビニのパンだけで昼ご飯をすませました。
    = I got by with only convenience‑store bread for lunch.
しか
- Must be used with a negative verb.
- Emphasizes limitation, often with a slightly negative or regretful nuance.
- Rough equivalent of “nothing but / no more than.”
- Example:
  - コンビニのパンしか食べませんでした。
    = I ate nothing but convenience‑store bread.

You can’t say しか with a positive verb (コンビニのパンしか食べました is wrong).

What does 昼ご飯を do in this sentence? Why is を used?

を marks the direct object of the verb. Here:

昼ご飯 = lunch
昼ご飯をすませました = (I) finished/dealt with lunch / got lunch over with

So:

コンビニのパンだけで = with only convenience‑store bread (means)
昼ご飯を = lunch (object)
すませました = (I) took care of / got over with

The structure is “I took care of lunch with only convenience‑store bread.”

What does すませました actually mean? How is it different from just 食べました?

すませました is the polite past form of すませる (or 済ませる), which here means:

to get something over with
to take care of something
to make do with something (for a certain purpose)

So:

昼ご飯をすませました。
≈ I took care of lunch / I got lunch over with.

Using コンビニのパンだけで adds the nuance “by making do with only convenience‑store bread.”

If you said:

コンビニのパンを食べました。
= I ate convenience‑store bread.

That’s a simple statement about what you ate.

コンビニのパンだけで昼ご飯をすませました。
= I managed / made do with only convenience‑store bread for lunch (implies it wasn’t a full proper meal or you just settled for that).

What is the dictionary form and plain past of すませました? Is this the same as 済ます?

すませました is:

Polite past of すませる (済ませる)

Forms:

Dictionary form: すませる
Polite non‑past: すませます
Plain past: すませた
Polite past: すませました

There is also a related verb 済ます (すます, a godan verb). In practice:

食事をすませる and 食事を済ます are both used.
In many set phrases for “take care of / finish (a task/meal),” ～をすませる/済ませる is extremely common.

In this sentence, you should think of it as すませる → すませました.

Why is past tense (すませました) used even though the sentence has 今日 (today)?

Japanese uses the past tense for actions that are already completed, even if they happened earlier today.

So if it’s now evening and you’re talking about lunch:

今日(の昼)はコンビニのパンだけで昼ご飯をすませました。
= Today, I (already) took care of lunch with only convenience‑store bread.

It doesn’t mean “in the past, not today”; it just shows the action is finished. This is completely natural.

Can I change the word order, like moving 昼ご飯 earlier in the sentence?

Yes, Japanese word order is relatively flexible as long as you keep particles attached to the right words. Some natural variations:

今日は昼ご飯をコンビニのパンだけですませました。
今日はコンビニのパンだけで昼ご飯をすませました。 (original order)
コンビニのパンだけで今日は昼ご飯をすませました。 (emphasizes “today” a bit differently)

All still mean essentially the same: you got by with only convenience‑store bread for lunch today.

What you generally keep is:

The verb at the end.
だけで close to the noun it restricts (コンビニのパン).

What nuance does コンビニのパンだけ have? Does it mean “only bread” (no drink, etc.)?

コンビニのパンだけ literally means “only bread from the convenience store.” It usually implies:

No other substantial food items for lunch (no bento, no side dishes, etc.).

Whether it excludes drinks is more about context and common sense; many people would still drink something with the bread. The main point is:

For the meal itself, you made do with just convenience‑store bread, nothing else substantial.

Is there a more casual or a more formal way to say this sentence?

Casual:

今日はコンビニのパンだけで昼ご飯をすませた。
Even more casual / colloquial: 今日はコンビニのパンだけで昼ご飯すませた。 (dropping を is possible in speech)

More formal / written:

本日はコンビニのパンだけで昼食を済ませました。
- 本日 instead of 今日
- 昼食 instead of 昼ご飯
- 済ませました with the kanji

The meaning is the same; the differences are tone and register.