ame no hi ha tosyokan ga itumo yori aite imasu.

の links two nouns, so 雨の日 literally means a rain day or a day of rain. In natural English, that becomes a rainy day or on rainy days.

This noun + の + noun pattern is very common in Japanese:

• 日本の会社 = a Japanese company
• 夏の日 = a summer day
• 雨の日 = a rainy day

So here, 雨 modifies 日.

The は marks 雨の日 as the topic or setting of the sentence.

So 雨の日は gives the sense of:

• as for rainy days...
• on rainy days...

It sets the condition under which the rest of the sentence is true. This is very natural in Japanese when talking about time, situations, or categories.

Because 雨の日は is already the topic, 図書館が is free to be marked as the grammatical subject of 空いています.

So the structure is roughly:

• 雨の日は = as for rainy days
• 図書館が = the library
• いつもより空いています = is less crowded than usual

Using が here helps identify the library as the thing that is in that state.

If you changed it to 図書館は, the nuance would shift a little, because then the library would also be treated as a topic. Japanese can sometimes have more than one は phrase, but this version with が is very straightforward.

Literally:

• いつも = always / usually
• より = than

So いつもより means than usual or than it usually is.

This is a very common comparison pattern in Japanese:

• Xより Y = Y is more/less ... than X

In this sentence, the comparison point is usual conditions.

It comes from 空いています together with いつもより.

空いています can describe a place as:

• empty
• not crowded
• having space

So when you say いつもより空いています, it means the place has more space / fewer people than usual. In natural English, that often becomes less crowded than usual.

Japanese does not need a separate word exactly matching English less here. The comparison is understood from the context.

Because the verb is 空く, which can be read in different ways depending on meaning.

Common readings include:

• あく = to open up / become available
• すく = to become less crowded / to thin out

Here the meaning is about the library having fewer people, so the reading is すいています.

Examples:

• 席が空いています can often be あいています = the seat is open/available
• 電車が空いています is usually すいています = the train is not crowded

So in this sentence, 図書館が空いています means the library is uncrowded, so すいています is the natural reading.

ている often describes a current state, not just an ongoing action.

With a verb like 空く meaning to become empty / to thin out, the ている form often means the result of that change:

• 空く = to become empty / uncrowded
• 空いている = to be empty / to be uncrowded

So 空いています describes the library's state: it is uncrowded.

This is a very common pattern with intransitive verbs in Japanese.

Most naturally, it sounds like a general statement about rainy days.

That is because:

• 雨の日は sounds like a category: on rainy days
• いつもより suggests a usual pattern or comparison

So the sentence feels like it is describing what tends to happen when it rains.

If the speaker wanted to focus on today, they might say something more specific, such as:

• 今日は雨なので、図書館はいつもより空いています。

That would make the sentence clearly about today.

Yes. Both are reasonable, depending on context.

空いています often covers ideas like:

• emptier
• less crowded
• more open
• having more space

For a place like a library, less crowded than usual is often the most natural English wording, because English speakers usually talk about how many people are there rather than whether the building is literally empty.

So the Japanese is flexible, and the best English translation depends on what sounds most natural in context.