kaado wo wasurenai you ni, saihu ni irete okimasu.

Because カード is the thing being forgotten in the clause カードを忘れないように.

In other words:

• 忘れる = to forget
• the thing you forget takes を

So:

• カードを忘れる = to forget the card

Even though that clause is not the main action of the sentence, it still follows normal grammar.

This is a very common pattern:

• verb-nai form + ように = so that... not / in order not to...
• verb dictionary form + ように can also mean so that... / in order to... in some cases

Here:

• 忘れない = not forget
• 忘れないように = so that I don’t forget / in order not to forget

It expresses a goal or precaution.

ように is very natural when talking about preventing something or making sure something does or does not happen.

So:

• 忘れないように = so I won’t forget / to make sure I don’t forget

This sounds like a precaution.

By contrast, ために is more often used for a clearer, more direct purpose with intentional actions. With forgetting, Japanese usually prefers ように because forgetting is not really something you intentionally do.

So 忘れないように is the standard and more natural choice here.

In ように, the に is part of the fixed grammar pattern.

So you should think of:

• ～ように as one set expression

It is not the same に as the one in 財布に later in the sentence.

Here, ように means something like:

• so that...
• in order to...
• to make sure...

Because に marks the destination or place something is put into.

• 財布に入れる = to put it into the wallet

So:

• カードを財布に入れる = to put the card in the wallet

Here に shows where the card ends up.

入れておきます is:

• 入れる = to put in
• ～ておく = to do something in advance / to do something and leave it that way

So 入れておきます means:

• I’ll put it in and leave it there
• I’ll put it in beforehand
• I’ll make sure it’s in there

This is more nuanced than just 入れます.

Compare:

• 入れます = I put it in / I will put it in
• 入れておきます = I’ll put it in ahead of time so it’s ready later

That fits the idea of preventing yourself from forgetting it.

Very often, yes, but it can also mean do something and leave it in that state.

In this sentence, both ideas work:

• put it in beforehand
• put it in and keep it there

So 財布に入れておきます suggests preparation for a future situation.

おきます is the polite non-past form of おく.

Japanese non-past can mean:

• present/habitual
• future intention

So here it could mean either:

• I’ll put it in my wallet so I won’t forget it
• I put it in my wallet so I won’t forget it (as a habit)

Without more context, both are possible, though many learners first understand it as a future/intended action.

The subject is omitted, which is very common in Japanese.

The sentence naturally implies something like:

• 私は = I
• sometimes 僕は, 私は, etc., depending on speaker and context

So the full idea is something like:

• (私は) カードを忘れないように、財布に入れておきます。

Japanese often leaves out subjects when they are obvious from context.

The comma helps separate the purpose/prevention part from the main action:

• カードを忘れないように、 = so I don’t forget the card,
• 財布に入れておきます。 = I’ll put it in my wallet.

It creates a natural pause.

The sentence could often be written without a comma too, but the comma makes it easier to read.

Yes, that is a very natural interpretation.

Because of 入れておきます, the sentence can suggest not just the act of placing it there, but also the resulting state of keeping it there as a precaution.

So depending on context, it can feel like:

• I’ll put the card in my wallet so I don’t forget it
• I keep the card in my wallet so I don’t forget it

Both reflect the same basic grammar and idea.

Japanese often expresses intentions through avoiding a bad result.

So instead of saying:

• so I can bring the card

it is very natural to say:

• so I don’t forget the card

Both are logical, but 忘れないように sounds very everyday and natural for this situation. It focuses on preventing the problem.