meisai wo yoku minai to, tiisana matigai ni kidukanaikoto ga arimasu.

Here, 明細 usually means an itemized statement or detailed breakdown.

Common examples:

• a bank statement
• a credit card statement
• a bill with line-by-line charges
• a pay statement

So in this sentence, it means something like the statement/details you should check carefully.

よく can mean different things depending on context.

Two very common meanings are:

• often
• well / carefully / closely

In 明細をよく見ないと, it means look carefully/closely, not look often.

So:

• よく見る = look closely, examine carefully
• not look at it frequently

That is a very common use of よく with verbs like 見る, 読む, 考える, etc.

見ないと is using the conditional と.

Breakdown:

• 見る = to look
• 見ない = do not look
• 見ないと = if you do not look / unless you look

So:

• 明細をよく見ないと
  = If you don’t look carefully at the statement... = Unless you look carefully at the statement...

A useful note: learners often first meet ないと in expressions like 行かないと meaning I have to go. But that is a shortened pattern where the rest is omitted. In this sentence, 見ないと is simply a normal if/unless clause because the sentence continues after it.

Because the two verbs take different particles.

• 明細を見る
  見る takes を for the thing you look at.
• 間違いに気づく
  気づく usually takes に for the thing you notice.

So:

• 明細をよく見ないと = if you don’t carefully look at the statement
• 小さな間違いに気づかない = don’t notice small mistakes

This is just how 気づく works in Japanese. Even though English says notice a mistake, Japanese normally says 間違いに気づく.

This pattern means there are times when... or it can happen that...

Breakdown:

• 気づかない = do not notice
• こと = turns the action into a noun-like idea
• があります = there are / it happens

So:

• 気づかないことがあります
  = there are times when you do not notice = you may fail to notice = it is possible not to notice

This makes the sentence sound more general and less direct than just saying:

• 気づきません = you do not notice

It suggests sometimes this happens, not this always happens.

Both are possible, but 小さな間違い is very natural.

About the difference:

• 小さい is a normal -い adjective
• 小さな is a special form used only before a noun

So:

• 小さい間違い = a small mistake
• 小さな間違い = a small mistake

The nuance is subtle, but 小さな often sounds a bit more descriptive or literary. In everyday Japanese, 小さな間違い is a very common phrase.

One important grammar point:

• You can say 間違いが小さい with 小さい
• But you cannot use 小さな by itself as a predicate

So:

• 小さな間違い ✔
• 間違いは小さなです ✘

Yes. Japanese often leaves the subject unstated when it is obvious or meant generally.

In this sentence, the implied subject is something like:

• you
• one
• people

So the sentence has a general meaning such as:

• If you don’t look carefully at the statement, you may not notice small mistakes.
• If people don’t check the details carefully, they may miss small errors.

The sentence is not necessarily talking about one specific person.

Because こと is not a person or animal. It is treated like a thing/event/occurrence.

Japanese uses:

• あります for things and abstract events
• います for people and animals

Here, 気づかないこと means the event/case of not noticing, so あります is correct:

• 気づかないことがあります = there are cases where you don’t notice

Yes:

• 明細 = statement / detailed breakdown
• を = object marker
• よく = carefully / closely
• 見ない = do not look
• と = if / unless
• 小さな = small
• 間違い = mistake
• に = marks what is noticed
• 気づかない = do not notice
• こと = nominalizes the action
• が = subject marker
• あります = there are / it happens

So the structure is:

• If you do not carefully look at the statement, there are times when you do not notice small mistakes.

More natural English:

• If you don’t look over the statement carefully, you may miss small mistakes.