kyonen no tetyou wo yomikaesitara, ryokou no omoide ga takusan kaite arimasita.

In this sentence, 手帳（てちょう） usually means a planner, datebook, or personal notebook/diary-like notebook.

It is not exactly the same as a plain ノート.

• ノート = an ordinary notebook
• 手帳 = a small book for schedules, notes, records, reminders, and sometimes short diary-style entries

So 去年の手帳 most naturally means last year’s planner/notebook.

The particle の connects nouns and shows a relationship between them.

Here it works like this:

• 去年の手帳 = the planner/notebook from last year
• 旅行の思い出 = memories of the trip / travel memories

So の often acts a bit like English of, 's, or an adjective-like connector between nouns.

読み返したら comes from the verb 読み返す（よみかえす）, which means:

• to read again
• to reread
• to look back through something you wrote or read before

So this is more specific than just 読む.

• 読んだら = if/when I read it
• 読み返したら = when I reread/looked back through it

In this sentence, the speaker is not reading the notebook for the first time. They are going back and reading it again.

In this sentence, たら means something like:

• when I did...
• after I did...
• and then I found...

So:

• 去年の手帳を読み返したら、…
  = When I looked back through last year’s planner, ...

This use of たら often introduces a discovery or something the speaker noticed after doing the first action.

It is not always a strong “if.” Very often in sentences like this, it means when/after.

This is one of the most important grammar points in the sentence.

The phrase is:

• 旅行の思い出がたくさん書いてありました

Here, 書いてある describes a state: something has been written down and is there.

So 思い出 is treated as the thing that exists in written form, which is why it takes が.

A rough structure is:

• X が 書いてある = X is written down / X has been written

Examples:

• 名前が書いてある = A name is written there.
• 予定が書いてある = The schedule is written there.

So 思い出が is natural because the sentence focuses on what was written there, not directly on the act of writing it.

書いてある means that something has been written and remains that way. It focuses on the resulting state.

Compare:

• 書いた = wrote
• 書いてある = is written / has been written down and is there

So:

• 旅行の思い出が書いてありました
  = Travel memories were written down there / There were travel memories written there

This wording draws attention to what the speaker found in the notebook, rather than simply saying someone performed the action of writing.

Both can sometimes be translated as was written, but they feel different.

書いてある

• focuses on the resulting state
• often implies someone wrote it intentionally
• very natural for notes, signs, schedules, diary entries, labels, etc.

書かれている

• passive form
• focuses more on the fact that something is written
• can sound more neutral, formal, or descriptive depending on context

In a notebook/planner context, 書いてある is very natural because the idea is:

the memories had been written down there and were still there in the notebook.

Not necessarily.

It only tells us that the writing existed there. In this kind of sentence, it is often understood that the speaker may have written it themselves in the past.

For example, the speaker rereads last year’s planner and finds that many travel memories are written in it. Most likely, they themselves wrote those notes earlier.

So 書いてありました does not automatically mean someone else wrote them.

思い出（おもいで） means memory or memories.

Japanese nouns usually do not change form for singular vs. plural. So 思い出 can mean:

• a memory
• memories

In this sentence, because of たくさん, it clearly means many memories.

So:

• 旅行の思い出 = memories of the trip / travel memories

たくさん means many / a lot.

Here it indicates that there were many memories written down.

So:

• 旅行の思い出がたくさん書いてありました

means that a lot of travel memories were written there.

You can think of たくさん as relating to the amount of 思い出.

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

A natural understanding is something like:

• I looked back through last year’s planner, and I found that many travel memories were written in it.

The sentence does not explicitly say I, but it is strongly implied.

Also, the notebook itself is the place where the memories were written, even though that location is not repeated in the second half because it is understood from context.

Because the whole sentence is being told in the polite style and in the past.

• ありました = polite past of ある
• 書いてありました = was written / had been written

The speaker is talking about what they found when they reread the notebook, so past tense is natural.

A plain-form version would be:

• 去年の手帳を読み返したら、旅行の思い出がたくさん書いてあった。

Yes, that is close to the grammar, but it sounds a little awkward in natural English.

A more natural English rendering would be something like:

• When I looked back through last year’s planner, I found lots of memories from my trip written in it.
• When I reread last year’s planner, I saw that I had written down lots of travel memories.

The Japanese focuses on the discovery of what was there in written form, so smooth English often adds words like I found, I saw, or written in it.