yokattara, kondo tosyokan de issyo ni nihongo wo benkyousimasen ka.

よかったら means something like if that’s okay with you, if you’d like, or if you’re interested.

It comes from よい / いい (good) and the conditional form 〜たら (if).
So literally, it is like if it is good, but in natural English it works as a soft, friendly lead-in before a suggestion or invitation.

In this sentence, it makes the invitation sound less direct and more polite:

• よかったら、今度…勉強しませんか。
  = If you’d like, shall we study Japanese together at the library sometime?

The comma shows a natural pause.

よかったら is being used as an introductory phrase, like:

• If you want,
• If that sounds good,
• By the way, if you're free,

So the comma helps separate that soft opener from the main invitation.

今度 often means next time, sometime soon, or on a future occasion, depending on context.

Here, it does not necessarily mean a specific scheduled next time. It is more like:

• sometime
• one of these days
• next time we get the chance

So 今度 図書館で suggests:

• at the library sometime
• next time, at the library
• sometime soon at the library

Japanese often leaves the exact timing vague unless it needs to be specific.

Because 今度 can function as an adverbial time expression, and many time expressions in Japanese do not need a particle.

For example:

• 今日行きます。 = I’m going today.
• 明日勉強します。 = I’ll study tomorrow.
• 今度会いましょう。 = Let’s meet sometime / next time.

So 今度 naturally modifies the sentence without needing に.

That said, some time expressions do use に, but 今度 usually does not.

The particle で marks the place where an action happens.

So:

• 図書館で = at the library
• literally, in the library as the location of the studying

This is different from に, which often marks:

• destination
• existence
• specific time
• indirect object, depending on context

Because 勉強する is an action taking place at a location, で is the correct particle here.

一緒 means together, but when it modifies the verb, it is commonly used as 一緒に.

The particle に helps turn it into an adverbial expression:

• 一緒に勉強する = to study together

This is very common in Japanese:

• 一緒に行きましょう。 = Let’s go together.
• 一緒に食べませんか。 = Won’t you eat together with me?

You may sometimes hear 一緒 in casual speech, but 一緒に is the standard form here.

Because 日本語 is the direct object of 勉強します.

• 日本語 = Japanese language
• を = marks the thing being studied
• 勉強する = to study

So:

• 日本語を勉強する = to study Japanese

This is a very standard verb-object pattern in Japanese.

This is one of the most common questions learners have.

In Japanese, negative question forms are often used to make invitations politely and softly.

So:

• 勉強しますか = Will you study? / Do you study?
  This sounds more like a question about whether someone will do it.
• 勉強しませんか = Won’t you study? / Would you like to study?
  This sounds like an invitation.

In natural English, we usually would not translate it literally as Won’t you study Japanese together with me? unless we want to sound unusual or dramatic. A more natural translation is:

• Would you like to study Japanese together?
• How about studying Japanese together?

Yes, it is polite.

It uses the ます form:

• dictionary form: 勉強する
• polite negative: 勉強しません
• polite invitation/question: 勉強しませんか

So this is appropriate for:

• classmates
• acquaintances
• coworkers
• people you are not extremely close to

A more casual version would be:

• 勉強しない？

The subject is omitted, which is very normal in Japanese.

From context, the sentence implies:

• you and I
• we

So the full idea is something like:

• If you’d like, shall we study Japanese together at the library sometime?

Japanese often leaves out words like:

• I
• you
• we

if they are obvious from context.

Because 一緒に already strongly suggests doing the action together, and the speaker’s involvement is understood from the invitation form 〜しませんか.

In English, we often say:

• Would you like to study with me?

In Japanese, it is usually natural to leave with me unsaid if the speaker is clearly the one inviting:

• 一緒に日本語を勉強しませんか。

If you wanted to be more explicit, you could say:

• 私と一緒に日本語を勉強しませんか。
  = Would you like to study Japanese together with me?

But in many situations, that is not necessary.

It can be both, depending on the rest of the sentence and the situation.

By itself, よかったら is fairly neutral and friendly. It often sounds soft and considerate rather than stiff.

Compare:

• よかったら、今度図書館で一緒に日本語を勉強しませんか。
  polite
• よかったら、今度一緒に勉強しない？
  casual

So よかったら works well in both polite and casual invitations.

Yes, 今度 can sometimes mean this time or next time, depending on context, which can be confusing.

Common possibilities:

• this time
• next time
• sometime soon

In this sentence, because it is an invitation about a future activity, 今度 is understood as:

• next time
• sometime soon

It would not normally be understood as this time here.

Yes. This sentence pattern is very flexible.

For example:

• よかったら、今度カフェで一緒に日本語を勉強しませんか。
  If you’d like, shall we study Japanese together at a café sometime?
• よかったら、今度オンラインで一緒に日本語を勉強しませんか。
  If you’d like, shall we study Japanese together online sometime?

The pattern is:

• よかったら、今度 [place] で 一緒に [object] を [verb] しませんか。

It is very natural and polite.

It sounds like a friendly invitation someone might say to:

• a classmate
• a coworker
• a friend they are not being extremely casual with
• a language exchange partner

Its tone is:

• soft
• considerate
• not pushy

That is largely because of:

• よかったら for softening
• 今度 for a relaxed, non-specific future time
• 勉強しませんか for a polite invitation

So overall, it is a very good model sentence for making polite suggestions in Japanese.