sekkaku kaigi no siryou wo insatusita noni, ie ni wasurete simaimasita.

せっかく means something like after going to the trouble of..., with effort, or finally doing something that took some work.

In this sentence, it emphasizes that printing the meeting materials was not effortless, so the speaker feels extra disappointed that they then left them at home.

A natural feeling is:

• I went to all that trouble to print the meeting materials, and then...

So せっかく adds an emotional nuance of wasted effort.

Here, のに means even though, despite the fact that, or and yet.

So:

• 印刷したのに = even though I printed them
• せっかく印刷したのに = even though I went to the trouble of printing them

This のに often expresses disappointment, frustration, or contradiction between expectation and reality.

The expected result would be:

• I printed the materials, so I should have brought them.

But the actual result is:

• I left them at home.

That mismatch is exactly what のに highlights.

会議の資料 means meeting materials or materials for the meeting.

The particle の connects two nouns. It often works like:

• A の B = B of A
• or B related to A

So here:

• 会議 = meeting
• 資料 = materials/documents
• 会議の資料 = materials for the meeting

Using を would not work here, because を marks the direct object of a verb, not a noun modifying another noun.

Japanese nouns usually do not change form for singular vs. plural.

So 資料 can mean:

• a document
• documents
• material
• materials

The exact number is understood from context.

In English, we often have to choose singular or plural, but in Japanese that distinction is often left unstated.

So 会議の資料 could naturally mean:

• the meeting materials
• the documents for the meeting

Because it comes before のに, the verb is often put in its plain form:

• 印刷したのに

This is normal grammar. Particles and grammar patterns like のに, けど, から, とき, etc. often attach to the plain form.

Even though the sentence ends politely with しまいました, a clause before のに does not need to match that politeness level.

So this combination is very common:

• 印刷したのに、忘れてしまいました。

It mixes:

• plain form inside the clause
• polite form at the sentence end

That is perfectly natural.

忘れました simply means forgot.

忘れてしまいました adds extra nuance. It often suggests:

• regret
• unfortunate completion
• oops / I ended up doing that
• sometimes accidentally

So:

• 忘れました = I forgot / I left it behind.
• 忘れてしまいました = I ended up forgetting it / I accidentally left it behind.

In this sentence, しまいました fits very well because the speaker is clearly unhappy about the result.

In Japanese, 忘れる can mean not only to forget mentally, but also to leave something behind somewhere.

So:

• 家に忘れる = to leave something at home

The particle に marks the place where the item was left.

So this part means:

• I left them at home
• I forgot them at home

It does not mean the speaker forgot about home. The omitted object is the materials.

The omitted object is 会議の資料.

Japanese often omits words that are already obvious from context. Since the materials were just mentioned, repeating them is unnecessary.

The full version could be:

• せっかく会議の資料を印刷したのに、会議の資料を家に忘れてしまいました。

But this sounds repetitive. So Japanese naturally leaves the object out:

• ...家に忘れてしまいました。

English usually needs the object stated more often, but Japanese often drops it when it is clear.

Here, に marks the location where something was left.

• 家に忘れる = leave it at home

Using で would not sound right here, because で usually marks the place where an action is actively performed.

Compare:

• 家で印刷した = printed it at home
• 家に忘れた = left it at home

So:

• で = where an action happens
• に = where something exists, arrives, is placed, or is left

Yes, せっかく and のに work very naturally together.

They each add a slightly different nuance:

• せっかく = I put in effort / it was a valuable chance / it feels wasteful
• のに = even though / and yet / contrary to expectation

Together, they strongly express:

• I went to all that trouble, and yet...

So the combination makes the disappointment more vivid than either one alone.

Yes, very much.

In many sentences, のに does more than just mean although. It often carries a feeling like:

• and yet
• what a shame
• despite that
• so frustrating

In this sentence, the speaker is not neutrally stating two facts. They are showing regret:

• I printed the materials.
• But unfortunately, I left them at home.

So のに helps express that emotional contrast.

A few natural translations are:

• Even though I went to the trouble of printing the meeting materials, I left them at home.
• I printed the meeting materials and then ended up leaving them at home.
• After going to all that trouble to print the meeting materials, I forgot them at home.

If you want to reflect しまいました more clearly, you could say:

• I accidentally left them at home.
• I ended up leaving them at home.

Yes. A more casual version might be:

• せっかく会議の資料を印刷したのに、家に忘れちゃった。

Here, 忘れてしまいました becomes 忘れちゃった, which is a casual contraction of 忘れてしまった.

This sounds more conversational and informal.

So:

• 忘れてしまいました = polite
• 忘れちゃった = casual

The meaning is basically the same, but the casual version sounds more like everyday spoken Japanese.