kono apuri ni roguinsitara, atarasii siryou wo daunroodo dekimasu.

Because ログインする usually treats the place/system you log into as the target marked by に.

• アプリにログインする = log in to the app
• サイトにログインする = log in to the site

Here, に shows the destination/target of the login. Using を would sound unnatural in this pattern.

したら is the たら form of する. In this sentence, it means if / when you log in.

So:

• ログインする = to log in
• ログインした = logged in
• ログインしたら = if/when you log in

This pattern often introduces a condition:

• Xしたら、Y = If/when X happens, Y

Here:

• このアプリにログインしたら、新しい資料をダウンロードできます。
• If/When you log in to this app, you can download the new materials.

It can feel like either, depending on context.

In this sentence, ログインしたら means something like:

• if you log in
• once you log in
• when you log in

The nuance is that logging in is the condition that allows the next action. In English, all three can work, but once you log in or if you log in often matches the feeling well.

ログイン is a borrowed noun, but in Japanese many borrowed nouns can combine with する to become verbs.

So:

• ログイン = login
• ログインする = to log in

Then that verb conjugates normally:

• ログインする
• ログインした
• ログインしたら

This is very common with loanwords:

• ダウンロードする = to download
• コピーする = to copy
• チェックする = to check

Both are possible, but ダウンロードできます is shorter and more natural here.

There are two related ways to express ability:

• ダウンロードできます
• ダウンロードすることができます

The second is more formal or wordy. In everyday Japanese, the shorter できます version is very common.

In this sentence, ダウンロードできます means can download.

Here, できます expresses ability or possibility.

So:

• ダウンロードできます = can download / it is possible to download

It is also the polite form of できる.

This sentence is not saying you will download; it is saying that after logging in, downloading becomes possible.

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

So this sentence does not explicitly say:

• you can download
• users can download
• one can download

But from the situation, the listener naturally understands who it applies to. In app instructions, omitted subjects are extremely common.

It could be either. Japanese nouns usually do not have separate singular and plural forms the way English does.

So 資料 can mean:

• a document
• documents
• materials

You understand the number from context. In this kind of sentence, 新しい資料 often means new materials or new documents.

資料 is a fairly broad word. It often means:

• materials
• documents
• reference materials
• handouts
• data

It is a little broader than just document. In an app or work/study context, 資料 often refers to files or materials you need to read or use.

新しい is an i-adjective, and i-adjectives go directly before nouns.

So:

• 新しい資料 = new materials

You do not need anything between them.

Examples:

• 新しい本 = a new book
• 新しいアプリ = a new app

The comma helps separate the condition from the main result:

• このアプリにログインしたら、
• 新しい資料をダウンロードできます。

It makes the sentence easier to read, especially because the first part sets up the condition. Japanese commas are often flexible, so sometimes you may see them, and sometimes not.

It uses the polite ending できます instead of plain できる.

Compare:

• ダウンロードできます = polite
• ダウンロードできる = plain/casual

App instructions, notices, and customer-facing text often use polite Japanese because it sounds more professional and friendly.

Yes, a version with 〜れば is possible:

• このアプリにログインすれば、新しい資料をダウンロードできます。

This also means if you log in to this app, you can download the new materials.

The difference is small here:

• 〜たら often feels a bit more like when/once that happens
• 〜れば is a straightforward conditional if

In this sentence, both work, but 〜たら sounds very natural for a condition that leads to the next step.