syoumeisyo ga itimai tarinakatta node, sinsei ha asita ni narimasita.

Because 足りない / 足りなかった often describes the thing that is lacking or insufficient, and that thing is commonly marked with が.

So:

• 証明書が足りなかった = the certificate(s) were insufficient / a certificate was missing
• more literally: As for certificates, there was not enough

Here, が marks what was lacking.

一枚（いちまい） is the counter for flat objects, including sheets of paper and many documents.

Since a certificate is a paper document, 枚 is the natural counter.

So:

• 証明書が一枚足りなかった = one certificate was missing
• literally: the certificates were short by one sheet/document

This is very natural Japanese.

This is a common pattern in Japanese:

• Noun + が + number/counter + verb

So:

• 証明書が一枚足りなかった

means roughly:

• Certificates were lacking by one

The counter phrase tells you how much was missing.

You could think of it like:

• 何が？ → 証明書
• どのくらい？ → 一枚
• どうだった？ → 足りなかった

足りなかった is the plain past-negative form of 足りる.

• 足りる = to be enough
• 足りない = is not enough
• 足りなかった = was not enough / was missing

This sentence mixes plain-style clauses with a polite ending:

• 足りなかったので
• 明日になりました

That is very normal in Japanese. A sentence often has:

• a plain form in the reason clause
• a polite form at the end

You could also say:

• 証明書が一枚足りませんでしたので、申請は明日になりました。

That sounds more fully polite, but the original is completely natural.

Yes — literally, 足りなかった means was not enough.

But in English, when talking about documents, we often translate that more naturally as:

• one certificate was missing
• we were missing one certificate
• we were short one certificate

So the Japanese expression focuses on insufficiency, while natural English often uses missing.

ので gives a reason or cause, similar to:

• because
• since

In this sentence:

• 証明書が一枚足りなかったので = because one certificate was missing

Compared with から, ので often sounds:

• a bit softer
• a bit more explanatory
• a bit more formal/polite in tone

So here ので fits well, especially in a situation involving paperwork or official procedures.

は marks 申請 as the topic.

So 申請は means something like:

• as for the application
• the application, on the other hand

This helps frame what changed:

• 申請は明日になりました = the application ended up being tomorrow / was moved to tomorrow

The topic marker is useful because the first part talks about the missing certificate, and then the sentence shifts to what happened to the application.

This is a very common Japanese use of なる.

• なる = to become
• but in scheduling/context, X になる often means it ended up being X or it was changed to X

So:

• 申請は明日になりました

means:

• the application was moved to tomorrow
• it will be tomorrow for the application
• the application ended up being tomorrow

It does not mean the application literally transformed into tomorrow. It means the planned time/date became tomorrow.

Here, に marks the result of the change with なる.

Pattern:

• A は B になる = A becomes B

So:

• 申請は明日になる = the application becomes tomorrow
  → more natural English: the application is moved to tomorrow

This is different from the normal time particle question. With many time words, 明日 often appears without に when simply meaning tomorrow:

• 明日申請します = I will apply tomorrow

But in 明日になる, the に is there because なる takes its result with に.

Yes, you can think of it as slightly compact.

Depending on context, it may imply something like:

• 申請の日程は明日になりました
• 申請するのは明日になりました
• 申請手続きは明日になりました

Japanese often leaves out words that are obvious from context. So 申請は明日になりました is natural even though English might prefer a fuller expression like:

• The application process has been postponed until tomorrow
• We’ll submit the application tomorrow instead

Japanese often omits the subject when it is understood from context.

So this sentence does not explicitly say:

• who noticed the problem
• who needed the certificate
• who will do the application

Depending on context, it could mean:

• we were missing one certificate
• they were missing one certificate
• one required certificate was missing

Japanese focuses more on the situation itself than on always stating the subject.

No, that would not be natural here.

With 足りる / 足りない, the thing that is sufficient or insufficient is usually marked by が, not を.

So the natural form is:

• 証明書が一枚足りなかった

Using を would sound wrong in this sentence.

No. In normal Japanese, this would usually be written without spaces:

証明書が一枚足りなかったので、申請は明日になりました。

Spaces are often added in learning materials to make the structure easier to see.