kyou ha ame datta node, itinitizyuu ie de hon wo yomimasita.

は marks the topic of the sentence. Here, 今日は means something like as for today or today, ....

So the sentence is framed around today:

• 今日は雨だったので... = Since it was rainy today...

Even though 今日 is a time word, it can still be made the topic with は.

雨 is treated like a noun in Japanese, so to say it was rain / it was rainy, you use the copula in the past:

• plain: 雨だった
• polite: 雨でした

In this sentence, 雨だった is used because it comes before ので, and Japanese often uses the plain form before connectors like ので even when the sentence ends politely.

So this pattern is very normal:

• 雨だったので、本を読みました。

Yes, in English rainy is an adjective, but Japanese does not always match English word classes.

雨 literally means rain, and Japanese commonly says:

• 雨だ = It is rain / It is rainy
• 雨だった = It was rainy

So even though English uses an adjective, Japanese uses a noun + copula here.

ので means because, since, or so. It gives a reason.

• 雨だったので、一日中家で本を読みました。
• Because it was rainy, I read a book / books at home all day.

Compared with から, ので often sounds a little softer, more explanatory, and a bit more neutral.

Both can mean because, but they have slightly different nuance.

• ので: softer, more explanatory, often sounds a bit more objective
• から: more direct, sometimes more personal or assertive

For example:

• 雨だったので、家にいました。 = Since it was rainy, I stayed home.
• 雨だったから、家にいました。 = also Because it was rainy, I stayed home.

Both are correct here, but ので feels a little gentler.

This is a very common pattern in Japanese.

In a sentence with multiple clauses, the final clause usually determines the overall politeness. Earlier clauses often stay in the plain form.

So:

• 雨だったので、読みました。

is perfectly natural.

Using polite forms in both places is possible:

• 雨でしたので、一日中家で本を読みました。

But that sounds more formal or stiff in everyday conversation.

一日中 means all day or the whole day long.

Breakdown:

• 一日 = one day
• 中 = throughout / during / all through

Together:

• 一日中 = throughout the whole day

So:

• 一日中家で本を読みました。 = I read at home all day.

Some time expressions can be used without a particle. 一日中 works adverbially by itself, so it can directly describe the action.

• 一日中本を読みました。 = I read all day.

This is very natural. You do not need に or another particle here.

Because で marks the place where an action happens.

• 家で本を読みました。 = I read a book at home.

Use に for things like:

• destination: 家に帰る = go home
• existence: 家にいる = be at home

Since reading is an action done in a location, で is the correct particle.

Japanese usually does not mark singular/plural as clearly as English does. So 本 can mean:

• a book
• books
• reading

The exact meaning depends on context. In this sentence, 本を読みました could naturally be understood as:

• I read a book
• I read books
• I spent the day reading

Japanese often leaves that detail unspecified.

を marks the direct object of the verb.

Here, the verb is 読む (to read), and the thing being read is 本.

So:

• 本を読む = to read a book / books

That is why 本 takes を.

Japanese often omits the subject when it is clear from context.

So this sentence does not explicitly say I, but in normal context it is understood as:

• I read at home all day because it was rainy today.

Depending on context, it could theoretically refer to someone else, but without extra information, learners are usually meant to understand it as I.

The basic idea can be expressed in other orders, but this order is very natural.

Current order:

• 今日は雨だったので、一日中家で本を読みました。

This puts the reason first, then the main action.

You could also say:

• 一日中家で本を読みました。今日は雨だったので。

But that sounds more like spoken style or afterthought. The original sentence is the most standard, smooth version.

Yes, that is possible, but the nuance is slightly different.

• 今日は雨だったので... clearly means because it was rainy today...
• 今日は雨で... can also connect the ideas, but it may sound a bit more like it was rainy, and... depending on context

If you specifically want to show a clear reason, ので is very useful.

The spaces are only there to help learners read the sentence more easily.

Normal Japanese writing would usually be:

• 今日は雨だったので、一日中家で本を読みました。

Japanese is normally written without spaces between words.