watasi ha itumo neru mae ni tetyou de asita no yotei wo kakuninsimasu

Because when a verb comes before 前 to mean before doing X, Japanese uses the dictionary/plain form of the verb.

• 寝る前に = before going to sleep
• Not 寝ます前に

This is a very common pattern:

• 食べる前に = before eating
• 行く前に = before going
• 勉強する前に = before studying

So 寝る前に is the normal grammar pattern.

前 means before, and 前に means before (an event/action).

In this sentence:

• 寝る前に = before going to bed / before sleeping

The に marks the time point before which the main action happens.

So the structure is:

• Verb (dictionary form) + 前に + main action

Example:

• 寝る前に本を読みます = I read a book before sleeping.

Here, に marks a point in time.

Japanese often uses に with specific times or time-related expressions when something happens at that point.

So in:

• 寝る前に

the に shows that the checking happens at the time before sleeping.

You will often see:

• 食事の前に = before the meal
• 出かける前に = before going out

In casual speech, に is sometimes dropped in some expressions, but 前に is the standard form learners should use.

は marks 私 as the topic of the sentence.

So 私は means something like:

• As for me, ...
• I ...

It does not simply mean subject in the same way English does. It introduces what the sentence is about.

In this sentence:

• 私はいつも寝る前に…確認します
• As for me, I always check ... before going to bed.

Also, in real Japanese, 私 is often omitted if it is already clear who is speaking. So a very natural version could simply be:

• いつも寝る前に手帳で明日の予定を確認します。

いつも means always, and adverbs like this are fairly flexible in Japanese.

In your sentence:

• 私は いつも 寝る前に 手帳で 明日の予定を 確認します。

this sounds natural and clear.

You could also hear:

• 私は寝る前にいつも手帳で明日の予定を確認します。

Both are understandable. The difference is mostly one of emphasis and rhythm, not basic meaning.

A good rule for learners:

• Put いつも before the part it modifies, or early in the sentence.

Here, で marks the means/tool used to do something.

So:

• 手帳で確認します = check using a planner / notebook

The idea is: the planner is the tool or medium used to confirm the schedule.

Compare:

• スマホで調べます = look it up with/on a smartphone
• ペンで書きます = write with a pen

So で here means something like using or by means of.

Usually, not in this sentence.

• 手帳で確認します = check using the planner
• 手帳に would suggest something involving in/on the planner, such as writing something there

For example:

• 手帳に予定を書きます = I write the schedule in my planner.
• 手帳で予定を確認します = I check the schedule using my planner.

So で is correct because the planner is the tool used for checking.

明日 means tomorrow, 予定 means schedule/plans, and の links them together.

So:

• 明日の予定 = tomorrow’s schedule
• or plans for tomorrow

Here, の works like a possessive or descriptive connector:

• 日本の車 = Japanese car / car of Japan
• 私の本 = my book
• 明日の予定 = tomorrow’s plans

It is very common for time words to modify nouns this way:

• 今日のニュース = today’s news
• 来週の会議 = next week’s meeting

Because 予定 is the direct object of 確認します.

• 確認します = check / confirm
• What do you check?
• 明日の予定を = tomorrow’s schedule

So を marks the thing being checked.

This is a standard object pattern:

• 本を読みます = read a book
• 映画を見ます = watch a movie
• 予定を確認します = check the schedule

確認します means more than just look at. It means:

• check
• confirm
• make sure of

So it suggests purpose: not merely seeing the schedule, but checking it carefully to make sure you know what is planned.

Compare:

• 予定を見ます = I look at the schedule.
• 予定を確認します = I check/confirm the schedule.

In many situations, 確認する sounds more natural when the goal is to make sure something is correct or remembered.

Yes. In Japanese, the non-past form can express:

• present
• future
• habitual actions

Here, because of いつも and the overall context, the sentence clearly describes a habitual routine:

• I always check tomorrow’s schedule before going to bed.

So 確認します does not only mean I will check. It can also mean I check as a regular habit.

Japanese word order is more flexible than English, especially before the final verb. The main verb usually comes at the end, but other parts can move around somewhat.

Your sentence:

• 私はいつも寝る前に手帳で明日の予定を確認します。

Other possible orders include:

• 私は寝る前にいつも手帳で明日の予定を確認します。
• 明日の予定を私はいつも寝る前に手帳で確認します。

These may sound more or less natural depending on context and emphasis, but they are grammatical.

For learners, the safest order is the original one:

• topic → time/frequency → time expression → tool → object → verb

Yes, very often.

Japanese frequently omits subjects and topics when they are obvious from context. If you are talking about your own routine, you can naturally say:

• いつも寝る前に手帳で明日の予定を確認します。

This still clearly means I always check tomorrow’s schedule in my planner before going to bed, if the speaker is understood.

Including 私 is not wrong, but it can sound a little more explicit than necessary unless you want contrast, such as:

• 私は確認しますが、弟はしません。
• I check it, but my younger brother doesn’t.