Learn Hardcore Japanese: isogasii hi ni ha, insutantoraamen dake de bangohan wo sumasemasu. - On busy days, I just have instant ramen for dinner.

You could also say 忙しい日は without に. That would be more like “Busy days, I…” without the explicit “on/when,” but the meaning is almost the same in everyday speech.

Why does 忙しい come before 日? What is the structure of 忙しい日?

忙しい is an i-adjective meaning busy. In Japanese, adjectives come directly before the noun they modify:

忙しい日 – a busy day
高いビル – a tall building
新しい本 – a new book

So 忙しい日には literally is:

忙しい (busy) + 日 (day) + に (on) + は (topic)
→ “On busy days, …”

What does だけで mean in インスタントラーメンだけで? Why not だけを?

だけ means only.
で here is the particle of means / material / “with / by / using”.

So:

インスタントラーメンだけで
→ with only instant ramen / using only instant ramen / just with instant ramen

If you said インスタントラーメンだけを, that would be more like:

(I) only (ever) eat instant ramen (restricting the object, not the means of the meal)

But this sentence is about what you use to “cover” dinner, not about the number of things you eat in general. So だけで晩ご飯をすませます means:

I get by for dinner with only instant ramen (as the meal).

The pattern XだけでYをすませる is common:
パンだけで昼ご飯をすませた。 – I made do for lunch with only bread.

What is the role of で in インスタントラーメンだけで晩ご飯をすませます?

Here, で marks the means/method or what serves as the dinner:

インスタントラーメンだけで – by means of only instant ramen / with only instant ramen
晩ご飯をすませます – (I) finish/settle my dinner / make do with dinner

A similar structure:

サンドイッチで昼ご飯をすませる。
– I make do for lunch with sandwiches.

So で tells you what is functioning as the dinner or the means by which the dinner is “taken care of.”

What does すませます mean exactly? Is it just “to eat”?

すませます is the polite form of すませる (also written 済ませる), meaning:

to finish something
to get something over with
to manage / make do (with something)

In this context, it’s closer to “to make do / get by (with)” than just “to eat”.

So:

晩ご飯をすませます
→ I take care of / get my dinner over with / make do for dinner (using X).

Combined with インスタントラーメンだけで:

インスタントラーメンだけで晩ご飯をすませます。
→ I just make do for dinner with instant ramen.

If you only wanted to say I eat instant ramen for dinner, you might say:

晩ご飯にインスタントラーメンを食べます。

What is the dictionary form of すませます, and is there a simpler word I could think of?

The dictionary (plain) form is すませる.

Polite: すませます
Plain: すませる

A near-synonym is すます; in many casual contexts 晩ご飯をすます and 晩ご飯をすませる feel similar (to finish / to make do with dinner).

You can remember すませる as:

to settle / finish / get something done,
often with the nuance of “not doing anything special, just covering the bare minimum.”

There is no I in the Japanese sentence. How do we know it means “I” make do with instant ramen?

Japanese often omits the subject when it’s obvious from context.

In a sentence like:

忙しい日には、インスタントラーメンだけで晩ご飯をすませます。

it is natural to understand the subject as the speaker:

(私は) 忙しい日には、インスタントラーメンだけで晩ご飯をすませます。
→ On busy days, I just make do with instant ramen for dinner.

If the context were about someone else, the subject would be understood as that person or explicitly stated:

母は忙しい日には、インスタントラーメンだけで晩ご飯をすませます。
– My mother makes do with just instant ramen for dinner on busy days.

The verb is in ます form (すませます). Does this mean present, future, or habitual action?

Japanese polite non-past (〜ます) can cover:

Present: what is generally true
Future: what will happen
Habitual: what usually happens

Here, with 忙しい日には (“on busy days”), the most natural reading is habitual:

On busy days, I (usually) just make do with instant ramen for dinner.

If you wanted to explicitly describe a one-time past event, you’d say:

忙しい日には、インスタントラーメンだけで晩ご飯をすませました。
– On (that) busy day, I made do with just instant ramen for dinner.

Can I say 晩ご飯はインスタントラーメンだけで(を)すませます? How would that change the nuance?

Natural versions:

晩ご飯はインスタントラーメンだけですませます。
(often people insert で after だけ here)
晩ご飯はインスタントラーメンだけです。 (simpler, without すませます)

Nuance differences:

晩ご飯をインスタントラーメンだけですませます。
– Focus on 晩ご飯を as the object being settled/finished.
晩ご飯はインスタントラーメンだけで(すませます)。
– 晩ご飯 is the topic:
“As for dinner, I (usually) make do with only instant ramen.”

Both are grammatical; changing を to は often shifts from “this is the thing being acted on” to “as for this, here is my comment.”

What is the difference between 忙しい日は and 忙しい日には? Are both correct?

Both are correct, and both are common.

忙しい日は、インスタントラーメンだけで晩ご飯をすませます。
→ Busy days, I make do with instant ramen for dinner.
忙しい日には、インスタントラーメンだけで晩ご飯をすませます。
→ On busy days, I make do with instant ramen for dinner.

日には has:

日 (day)
に (on that day)
は (as for)

So 日には gives a slightly clearer “on those days / when it’s a busy day” feeling.
日は sounds more like simply “busy days” as a topic. In practice, the difference is subtle and both are natural in conversation.

What is 晩ご飯 exactly? Is it the same as other “dinner” words like 夕ご飯 or 夜ご飯?

晩ご飯 (ばんごはん) means dinner / evening meal.

Common variations:

晩ご飯 – dinner (very common, neutral)
夕ご飯 (ゆうごはん) – also dinner/supper; some people prefer this
夜ご飯 (よるごはん) – colloquial; also “night meal,” often just dinner

In most everyday contexts, 晩ご飯 and 夕ご飯 can be used interchangeably to mean dinner. Your sentence would also be natural as:

忙しい日には、インスタントラーメンだけで夕ご飯をすませます。

Could I use しか〜ない instead of だけ here? For example, インスタントラーメンしか〜?

You could make a similar sentence with しか〜ない, but the structure changes:

インスタントラーメンしか食べません。
– I don’t eat anything but instant ramen.

しか〜ない puts the negation in the verb and emphasizes exclusivity.
In your original sentence, the pattern is:

XだけでYをすませる – to make do with only X for Y.

A roughly parallel しか sentence might be:

忙しい日には、晩ご飯にインスタントラーメンしか食べません。
– On busy days, for dinner I eat nothing but instant ramen.

Both convey “only instant ramen,” but:

だけで晩ご飯をすませます sounds like “I get by with only instant ramen for dinner” (focus on making do).
インスタントラーメンしか食べません is “I eat nothing but instant ramen” (focus on restriction).