densya wo nagaku matte iru dake da to taikutu nanode, watasi ha tetyou wo yomikaesimasu.

長い is an i-adjective, and when an i-adjective modifies a verb, it usually changes to its -く form.

• 長い = long
• 長く待つ = to wait for a long time

So 電車を長く待っている means waiting a long time for the train.

This is similar to:

• 早い → 早く行く = go quickly/early
• 短い → 短く切る = cut short

Yes — in English we say wait for the train, but in Japanese 待つ often takes a direct object with を.

So:

• 電車を待つ = wait for the train
• 友だちを待つ = wait for a friend

This is just a difference between Japanese and English grammar. Even though English uses for, Japanese commonly uses を with 待つ.

Here 待っている means is waiting / am waiting — the ongoing action of waiting.

With many verbs, ～ている shows:

1. an action in progress
   
   • 食べている = is eating
2. a continuing state resulting from something
   
   • 結婚している = is married

In this sentence, 待つ is being used in the first way: an ongoing action.

So 電車を長く待っている means being in the situation of waiting a long time for the train.

だけ means only, just, or nothing but.

So:

• 待っているだけ = just waiting / doing nothing but waiting

It adds the nuance that no other activity is happening.

Compare:

• 電車を待っている = waiting for the train
• 電車を待っているだけ = just waiting for the train, and not doing anything else

That idea leads naturally into it’s boring, which is why the next part makes sense.

The だ is there because だけ by itself is not acting like a complete predicate here. The speaker is treating 待っているだけ as a statement: it is only waiting / if it’s just waiting.

So:

• 待っているだけ = just waiting
• 待っているだけだ = it’s just waiting / that’s all it is
• 待っているだけだと = if it’s just waiting / when it’s only waiting

You can think of だと here as attaching the conditional と to the idea expressed by だけだ.

This is a very common pattern:

• 一人だと寂しい = If I’m alone, it’s lonely
• 安いだけだと不安だ = If it’s only cheap, that makes me uneasy

Here と is a conditional, roughly meaning if or when.

So:

• 待っているだけだと退屈 = if you’re just waiting, it’s boring

This と often expresses something that naturally happens or tends to happen when the first condition is true.

In this sentence, the idea is:

• If all I do is wait for the train for a long time, it gets boring.

So it connects the situation to its natural result.

Both ので and から can mean because, but ので often sounds a bit softer, more explanatory, and sometimes more formal.

• 退屈なので = because it’s boring / since it’s boring
• 退屈だから = because it’s boring

In this sentence, ので gives a smooth explanatory link:

• Since just waiting is boring, I reread my planner/notebook.

A learner can usually think of ので as a gentler because.

は marks 私 as the topic.

• 私は = as for me / I

The sentence is talking about what I do in response to the boring situation:

• 私は手帳を読み返します = I reread my planner/notebook

Japanese often uses は when setting up the person whose action or viewpoint is being discussed.

Also, 私 could be omitted if the context already made the subject obvious. It appears here for clarity or emphasis.

読み返す means to read again, to reread, or sometimes to look back over what you’ve read/written.

It is made from:

• 読む = read
• 返す = return/back

So 読み返します suggests going back and reading something again, not just reading it for the first time.

Compare:

• 手帳を読みます = I read my planner/notebook
• 手帳を読み返します = I reread / look back through my planner/notebook

That nuance fits well if the speaker is passing time by reviewing notes, plans, or entries already written there.

手帳 usually means a small notebook, planner, datebook, or pocket agenda.

Depending on context, it could be:

• a schedule book
• a notebook for personal notes
• a diary-like planner

So 手帳を読み返します suggests the speaker is looking back through their planner or notebook while waiting.

It is not usually the same as a full-sized notebook used for school notes; it often has a more personal, portable feel.

The comma separates:

1. the reason/background
   • 電車を長く待っているだけだと退屈なので
2. the main action
   • 私は手帳を読み返します

So the structure is basically:

• Because/Since X, I do Y
• If it’s only X, it’s boring, so I do Y

This is a very common Japanese sentence pattern: explain the situation first, then give the main action.

It can feel like a specific habitual or present situation, depending on context.

Because the sentence uses 読み返します in the non-past polite form, it can mean things like:

• I reread my planner (as a usual habit in that situation)
• I’ll reread my planner / I do reread my planner in this situation

And because 待っているだけだと退屈 is a fairly general observation, the whole sentence can sound like:

• When I have to wait a long time for the train and all I’m doing is waiting, it’s boring, so I reread my planner.

Japanese non-past often covers both habitual and near-present/future meanings, so context decides the exact reading.