kono apuri ha roguinsinai to tukaemasen si, tukaiowattara roguautosuru hituyou ga arimasu.

は marks このアプリ as the topic: As for this app...

That fits the sentence well because the rest of the sentence explains rules about the app:

• you cannot use it unless you log in
• you need to log out when you are done

If you used が instead, the nuance would be more like identifying or singling out this app in particular. For general instructions, は is very natural.

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

In this kind of instruction, the understood subject is the user, so English naturally supplies you:

• you must log in
• you need to log out

Japanese does this constantly, especially in instructions, rules, and manuals.

It is built like this:

• ログインする = to log in
• ログインしない = not log in
• ログインしないと = if you do not log in

So the structure is literally if you don’t log in, you can’t use it.

In natural English, that often becomes unless you log in, you can’t use it. So ないと here is a negative condition.

Related, but not the same function here.

Sometimes Japanese speakers say things like:

• 行かないと。
• 勉強しないと。

That is short for something like 行かないといけない or 勉強しないといけない, meaning I have to go / I have to study.

But in your sentence, ログインしないと is a real conditional because it is followed by a result:

• ログインしないと使えません
• If you don’t log in, you can’t use it

So here it is not an ellipsis meaning have to.

Because 使えません is the negative potential form:

• 使う = use
• 使える = can use
• 使えません = cannot use

By contrast:

• 使いません = do not use / will not use

So ログインしないと使えません means if you don’t log in, you cannot use it, not if you don’t log in, you will not use it.

They are both conditional forms, but they are used differently.

と

In ログインしないと使えません, と works well for a rule or automatic consequence:

• if you don’t log in, you can’t use it

It sounds like a system condition.

たら

In 使い終わったら, たら means when / after

• when you have finished using it
• after you’re done using it

So the sentence uses:

• と for a fixed condition or rule
• たら for what to do after a completed action

し links one point to another.

Here it connects:

• you can’t use it unless you log in
• you need to log out when you’re done

So it works a lot like and, but it often has a nuance of adding another relevant point:

• and also
• plus
• what’s more

It can make the sentence sound a little smoother or softer than two totally separate statements.

It comes from the compound verb 使い終わる, which means to finish using.

Breakdown:

• 使う = to use
• 終わる = to finish
• 使い終わる = to finish using
• 使い終わった = finished using
• 使い終わったら = when/after you have finished using

So this is a very common pattern:

• verb stem + 終わる = finish doing that action

Examples:

• 読み終わる = finish reading
• 食べ終わる = finish eating

This uses the pattern:

• dictionary form + 必要がある
• meaning there is a need to do...

So:

• ログアウトする必要があります
• literally: there is a need to log out
• natural English: you need to log out

This is a common formal way to state necessity in instructions and notices.

Because ログアウト is a verbal noun, often called a suru-verb noun.

That means:

• ログアウト = logout
• ログアウトする = to log out

Before 必要, Japanese uses the plain dictionary form of the verb:

• ログアウトする必要 = the need to log out

You cannot just drop する here, because the sentence needs the verbal idea to log out, not just the noun logout.

Because 必要がある is a set pattern.

Literally, it is something like:

• a need exists

So が marks 必要 as the thing that exists.

This is just the normal grammar of the expression:

• 必要がある = there is a need
• 必要がない = there is no need

You may also see 必要はありません, where は adds contrast or emphasis, but the basic pattern is 必要がある.

Because in Japanese, politeness is usually shown in the main clause, while verbs inside subordinate structures often stay in plain form.

Here:

• ログインしない is inside a conditional
• 使い終わったら is also a conditional expression
• ログアウトする modifies 必要

The main statements are:

• 使えません
• あります

Those are polite forms, so the whole sentence is polite.

This is completely normal Japanese. It is not a mix-up.

Yes. For example:

• 使い終わったら、ログアウトしなければなりません。

That would also mean you must log out when you are done.

The difference is mostly nuance:

• 必要があります sounds a bit objective and instruction-like
• しなければなりません sounds more directly like an obligation

In app instructions, 必要があります is very common.

They are loanwords that behave as verbal nouns.

That means they can combine with する to act like verbs:

• ログインする = to log in
• ログアウトする = to log out

This is very common with borrowed words in Japanese, especially technical ones:

• ダウンロードする = to download
• コピーする = to copy

So in your sentence, they are being used as verbs through する.

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

So this would normally be written as:

このアプリはログインしないと使えませんし、使い終わったらログアウトする必要があります。

The spaces were almost certainly added to make it easier for learners to read.