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Because 書いた日記 is a noun-modifying phrase meaning the diary entry I wrote or the written diary.

In Japanese, a plain-form verb can directly modify a noun:

• 書く日記 = a diary that someone writes / a diary one writes
• 書いた日記 = a diary entry that was written

So here, 書いた is not functioning as the main past-tense verb of the sentence. It is simply describing 日記.

Yes. Japanese uses relative clauses very naturally, and they come before the noun they describe.

So:

• 書いた日記 = the diary that I wrote
• literally, something like wrote diary

Japanese does not need relative pronouns like that, which, or who here.

The particle を marks 日記 as the direct object of 保存して.

So:

• 書いた日記を保存して = save the diary entry I wrote

Even though 保存して is in the て-form, it is still a verb, and 日記 is what gets saved.

すぐ means right away, immediately, or soon.

Here it modifies 保存して:

• 書いた日記をすぐ保存して = save the diary entry right away

Japanese adverbs like すぐ are often placed before the verb they modify.

The て-form here connects actions:

• save it right away
• and then add things remembered later

So 保存して、...追加する links the two actions in sequence.

In this sentence, the whole connected action is part of what the speaker habitually tries to do:

• save the written diary right away,
• and later add things they remembered.

思い出したこと means things I remembered or what I remembered.

Breakdown:

• 思い出す = to remember / to recall
• 思い出した = remembered
• こと = thing, matter, fact

So 思い出したこと literally means the things that I remembered.

This is another noun-modifying structure:

• 思い出した modifies こと

Because 追加する needs something to act on—an object. こと turns the idea into a noun-like expression.

Compare:

• 思い出した = remembered
• 思い出したこと = the thing(s) I remembered

Then:

• 思い出したことを追加する = add the things I remembered

Without こと, the phrase would not work as naturally as the object of 追加する.

あとで means later or afterward.

It comes from:

• あと = after, later
• で = particle used in this fixed time expression

As a set phrase, あとで is very common and simply means later:

• あとで追加する = add later

You usually learn it as a chunk rather than analyzing で too deeply every time.

〜ようにしている means something like:

• make a point of ...
• try to ... regularly
• make it a habit to ...

So:

• 追加するようにしています = I make it a habit to add ...
• and in this sentence, it applies to the routine described overall

This pattern often expresses repeated effort or a personal rule:

• 毎日運動するようにしている = I try to exercise every day
• 夜はコーヒーを飲まないようにしている = I make a point of not drinking coffee at night

〜ようにしています emphasizes an ongoing habit or continued effort.

Compare:

• 〜ようにします = I will try to ..., I’ll make sure to ...
• 〜ようにしています = I am making a habit of ..., I try to ... regularly

So しています shows this is not a one-time decision but a continuing practice.

In natural reading, it applies to the overall routine:

• save the diary right away
• and later add what I remembered

Even though ようにしています comes right after 追加する, the earlier て-form clause is understood as part of the same habitual pattern.

So the sentence means something like:

• I make it a habit to save diary entries right away and add things I remember later.

Because Japanese often omits subjects when they are understood from context.

Here, the sentence is about the speaker’s own habit, and 〜ています makes that very natural. So 私は is unnecessary unless the speaker wants emphasis or contrast.

A full version could be:

• 私は、書いた日記をすぐ保存して、思い出したことをあとで追加するようにしています。

But in normal Japanese, leaving out 私は sounds perfectly natural.

Usually, yes. 保存する often means to save in the sense of saving data, a file, a draft, and so on.

So 日記を保存する strongly suggests:

• a digital diary
• a note app
• a blog draft
• some electronic text

If it were a paper diary, Japanese would more naturally use other verbs depending on context rather than 保存する.

It is natural and understandable. It has a very common pattern:

• [action 1] て、[action 2] ようにしている

This lets the speaker describe a personal routine or habit involving multiple steps.

The sentence as a whole expresses:

• first saving the diary entry right away,
• then later adding any newly remembered details,
• and doing this as an ongoing habit.