sekkaku zikan ga aru kara, tosyokan de benkyousimasyou.

In this sentence, せっかく means something like since we have this good opportunity or since we’ve got this valuable chance.

It often adds the feeling of:

• this situation is special or fortunate
• it would be a shame not to make use of it

So せっかく時間があるから is not just because we have time. It has more of a nuance like:

• since we actually have some time
• since we’ve got this chance
• since we went to the trouble of making time

That emotional nuance is important.

They do overlap a little, but they do different jobs.

• せっかく adds the nuance of a valuable opportunity
• から gives the grammatical reason: because / since

So:

• せっかく = the speaker’s attitude toward the situation
• から = the actual reason marker

That means せっかく時間があるから、図書館で勉強しましょう。 feels like:

• Since we have this nice opportunity of having time, let’s study at the library.

Without せっかく, it would be more neutral:

• 時間があるから、図書館で勉強しましょう。
• Since we have time, let’s study at the library.

Japanese often expresses this idea as time exists rather than I have time.

So:

• 時間がある = literally time exists
• natural English meaning = have time / there is time available

This is a very common Japanese pattern. Instead of saying I have X, Japanese often says X exists.

For example:

• お金がある = I have money
• 時間がある = I have time
• 予定がある = I have plans

The person who has the time is understood from context, so Japanese usually doesn’t need to say 私たちは or 私は.

Because ある is an existence verb, and the thing that exists is usually marked with が.

So:

• 時間がある = there is time / time is available

This is the standard pattern.

A learner might wonder whether 時間はある is possible. It is, but the nuance is different:

• 時間がある = neutral statement of existence
• 時間はある = contrastive or emphasis, like as for time, we do have it

So in this sentence, が is the normal and natural choice.

Because 時間 is not a living thing.

Japanese uses:

• ある for non-living things and abstract things
• いる for people and animals

So:

• 時間がある = correct
• 時間がいる = incorrect

Even though time is abstract, Japanese still treats it like the kind of thing that takes ある.

から marks the reason or cause: because / since.

Here it connects:

• せっかく時間がある = the reason
• 図書館で勉強しましょう = the suggestion/result

So the structure is:

• [reason] から、[let’s do something]

This is a very common pattern in Japanese.

In this sentence, から works especially well because the second half is a suggestion: let’s study.

A softer alternative is ので, but から is very common and natural here.

Because で marks the place where an action happens.

Here, the action is 勉強する (to study), and that action takes place at the library.

So:

• 図書館で勉強する = study at the library

By contrast, に usually marks:

• destination
• point in time
• indirect object
• place of existence with ある / いる

So:

• 図書館に行く = go to the library
• 図書館で勉強する = study at the library

That’s why で is correct here.

It is not a command. It is the polite volitional form, and here it means:

• let’s study
• shall we study

It includes the speaker and the listener.

So 勉強しましょう is a polite suggestion or invitation.

Compare:

• 勉強する = study
• 勉強します = will study / study (polite)
• 勉強しよう = let’s study (plain)
• 勉強しましょう = let’s study (polite)

So this sentence sounds like the speaker is politely proposing a plan.

Japanese often leaves out subjects when they are clear from context.

In this sentence:

• the person or people who have time are understood from the situation
• 勉強しましょう strongly suggests the speaker and listener together

So English often needs a subject like we, but Japanese does not.

A natural English interpretation would be something like:

• Since we have time, let’s study at the library.

But the Japanese sentence does not need to explicitly say 私たち.

Yes. せっかく is very common, and it often appears when someone wants to make good use of a special situation.

Typical patterns include:

• せっかく〜から… = since we have this opportunity, ...
• せっかくだから… = since we’re at it / since we have the chance, ...
• せっかくの〜 = precious / special ...

Examples:

• せっかく日本に来たから、寿司を食べましょう。
  Since we came all the way to Japan, let’s eat sushi.

• せっかくだから、写真を撮りましょう。
  Since we have the chance, let’s take pictures.

So this sentence is using a very practical, common kind of せっかく.

No. Normal Japanese is usually written without spaces between words.

So this sentence would normally appear more like:

せっかく時間があるから、図書館で勉強しましょう。

The spaces are usually added only for learners, to make the parts easier to see.

So if you see spaces in study materials, that is normal for teaching, but not normal in everyday Japanese writing.