wasurenai you ni, genkan no doa ni memo wo hatte okimasu.

Because the form before ように is usually the plain form of the verb, not the polite ます form.

• 忘れない = plain negative
• 忘れません = polite negative

So:

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

Even though the main sentence ends politely with 貼っておきます, the earlier clause still uses the plain form because that is how this grammar pattern works.

Here, ように means something like so that or in order that.

In this sentence:

• 忘れないように = so that I won’t forget

So the first part gives the purpose or goal, and the second part tells the action taken for that purpose:

• 忘れないように、メモを貼っておきます。
• I’ll put up a note so that I don’t forget.

Because the speaker’s goal is to avoid forgetting.

Compare:

• 忘れる = to forget
• 忘れない = not to forget

So 忘れないように literally means so that (I) do not forget.

This is a very common pattern in Japanese:

• 遅れないように = so as not to be late
• 間違えないように = so as not to make a mistake
• 忘れないように = so as not to forget

A learner may wonder about that, but ように is the more natural choice here.

• 忘れないように = natural, common
• 忘れないために = possible in some contexts, but less natural here

A useful guideline:

• ために often sounds more like a direct, deliberate purpose: in order to
• ように is very common with:
  • negative forms
  • ability/potential expressions
  • goals like so that something will happen / not happen

Since forgetting is something the speaker wants to prevent, 忘れないように sounds very natural.

貼っておきます is 貼る in the ておく pattern.

• 貼る = to stick, attach
• 貼っておく = to stick something on in advance / to leave it stuck there for future convenience
• 貼っておきます = polite form

So this is not just I stick a note on the door. It has the nuance of:

• I’ll put up a note ahead of time
• I’ll leave a note there so it will be there when I need it

That fits the idea of preparing for later so the speaker does not forget.

The nuance is doing something beforehand or doing something and leaving it that way for later use.

In this sentence:

• メモを貼っておきます means something like
• I’ll stick a memo on the door and leave it there
• I’ll put up a note in advance

This is different from just 貼ります, which would simply mean I’ll stick it on without emphasizing preparation or leaving it in place for a purpose.

They are doing two completely different jobs.

1. 忘れないように

   • here に is part of the grammar pattern ように
   • it means so that / in order that

2. ドアに

   • here に marks the place/target where the memo is attached
   • ドアにメモを貼る = to stick a memo on the door

So even though the same particle appears twice, the function is different each time.

Yes, ドア already means door, but 玄関のドア is more specific.

• 玄関 = entranceway / front entrance
• 玄関のドア = the entrance door / front door

The speaker is specifying which door the memo will be attached to. That makes sense, because a memo on the front door is easy to notice when leaving.

玄関 refers to the entrance area of a home or building in Japanese culture. It is not just the physical door itself, but the entry space around it.

So:

• 玄関 = entryway / entrance area
• 玄関のドア = the door at the entrance, often the front door

This is a very common household word in Japanese.

Yes, it is natural.

• メモ = memo, note, reminder
• 貼る = to stick, attach

So メモを貼る means to stick up a note.

If the note were more like a posted notice or sign, Japanese might also use words like 張り紙. But here メモ sounds natural because it is a personal reminder.

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

From the context and the verb ending 貼っておきます, we understand that the speaker means:

• I’ll put a note on the front door so I don’t forget.

Japanese often leaves out words like I, you, or it when they are obvious from context.

Because the sentence is in polite style.

• 貼っておく = plain
• 貼っておきます = polite

So the speaker is speaking politely, but the earlier grammar still uses the plain form:

• 忘れないように、玄関のドアにメモを貼っておきます。

This mix is normal:

• subordinate clauses often use plain forms
• the main sentence ending shows the overall politeness level

The comma simply helps separate the purpose phrase from the main action.

• 忘れないように、 = so that I don’t forget,
• 玄関のドアにメモを貼っておきます。 = I’ll stick a memo on the front door.

It makes the sentence easier to read, but the grammar would still be understandable without it.

Yes, to some extent. Japanese word order is flexible as long as the particles make the relationships clear.

For example, this sentence could also be understood if parts were rearranged. But the given order is very natural because it presents:

1. the purpose: 忘れないように
2. the action: 玄関のドアにメモを貼っておきます

That order is especially common when the speaker wants to highlight why they are doing something.