gogo ni zikan ga dekitara, tosyokan ni yorimasu.

In this sentence, できる means to come into existence / to become available.

So 時間ができたら literally means:

• if time becomes available
• if some free time opens up

This is very natural Japanese. With 時間, できる often means to get free time.

Compare:

• 時間ができたら = if I end up having some free time / if time opens up
• 時間があったら = if I have time

The first one often suggests that the speaker is currently busy, but free time might appear later.

Because できる is an intransitive verb here.

The thing that comes into existence or becomes available is marked with が:

• 時間ができる = time becomes available
• 予定ができる = a plan gets made / a schedule is set
• 用事ができる = something comes up

If you used を, it would suggest a direct object, but できる does not work that way in this meaning.

A useful comparison:

• 時間ができたら = if free time opens up
• 時間を作れたら = if I can make time

Both can be natural, but they are slightly different in nuance.

〜たら is a conditional form that often means if or when.

Here:

• できる → past plain form できた
• できた + ら → できたら

So:

• 時間ができたら、図書館に寄ります。
• If I have time / If some time opens up, I’ll stop by the library.

In this kind of sentence, 〜たら often gives a natural sequence:

• first, time becomes available
• then, I stop by the library

So it can feel like if/when that happens, then...

The に marks the time when something happens.

So:

• 午後に = in the afternoon

With time expressions, に is often used to show the time reference. However, with broader time words, Japanese can sometimes omit it.

So both of these can be natural:

• 午後に時間ができたら、図書館に寄ります。
• 午後、時間ができたら、図書館に寄ります。

The version with に sounds a little more explicitly tied to the time phrase. The version without it can sound a bit lighter or more conversational, especially with a pause.

Because 寄る commonly takes the place you stop by with に.

• 図書館に寄る = to stop by the library
• コンビニに寄る = to stop by the convenience store
• 駅に寄る = to stop by the station

Here, に marks the destination/place of the stop.

This is different from English, where we say stop by the library, but in Japanese the place is often marked with に.

寄ります means more than just go.

It suggests:

• stopping by somewhere briefly
• dropping in
• making it one stop among other activities
• often going there on the way or as an extra visit

So:

• 図書館に行きます = I will go to the library
• 図書館に寄ります = I will stop by the library

寄る sounds lighter and more casual in purpose than 行く. It often implies that the library is not the main event, but a place the speaker will visit if the chance comes up.

In Japanese, the non-past form is used for both:

• present
• future

So 寄ります can mean:

• I stop by
• I will stop by

The future meaning comes from context. In this sentence, the conditional 時間ができたら clearly shows that the action has not happened yet, so 寄ります is naturally understood as future:

• If I have time in the afternoon, I’ll stop by the library.

This is completely normal Japanese.

Japanese very often leaves out the subject when it is obvious from context.

In this sentence, the person doing 寄ります is naturally understood to be the speaker, so I does not need to be said.

That is why:

• 午後に時間ができたら、図書館に寄ります。

already naturally means:

• If I have time in the afternoon, I’ll stop by the library.

You could add 私は if needed:

• 私は、午後に時間ができたら、図書館に寄ります。

But in many situations, that would sound a bit more explicit than necessary.

Yes, and the meaning would be similar, but the nuance changes.

• 時間があったら = if I have time
• 時間ができたら = if some time opens up / if I end up with free time

あったら is more general.

できたら suggests that free time may appear because the schedule changes, something finishes, or an opening becomes available.

So:

• 午後に時間があったら、図書館に寄ります。 = If I have time in the afternoon, I’ll stop by the library.

• 午後に時間ができたら、図書館に寄ります。 = If some time opens up in the afternoon, I’ll stop by the library.

The original sentence feels slightly more natural if the speaker expects to be busy but might become free.

The comma is helpful, but not strictly required.

• 午後に時間ができたら、図書館に寄ります。
• 午後に時間ができたら図書館に寄ります。

Both are acceptable.

The comma simply makes the sentence easier to read by showing the break between:

• the condition: 午後に時間ができたら
• the main action: 図書館に寄ります

Japanese punctuation is often more flexible than English punctuation, so this comma is mainly for clarity and rhythm.