syasin ga ooi node, daunroodo ga owaru made matimasu.

In this sentence, が marks the thing being described or the thing that does the action.

• 写真が多い = there are many photos / the photos are many
• ダウンロードが終わる = the download finishes

So:

• 写真が多いので = because there are many photos
• ダウンロードが終わるまで = until the download finishes

In English, we often use different structures, but in Japanese が is very common in these kinds of clauses.

Both are possible, but they have slightly different nuances.

• 写真が多い is a more neutral statement: there are many photos
• 写真は多い puts contrast or topic emphasis on photos, as in as for the photos, there are a lot of them

In this sentence, が sounds natural because the speaker is simply giving the reason: because there are many photos.

ので means because or since. It gives a reason.

• 写真が多いので、待ちます。 = Because there are many photos, I’ll wait.

Compared with から, ので usually sounds a bit softer and more explanatory.

• ので = more neutral, polite, explanatory
• から = more direct

So here, ので fits well.

Yes, 多い is an i-adjective, and it means many or a lot.

Japanese often uses 多い directly with a noun marked by が:

• 人が多い = there are many people
• 車が多い = there are many cars
• 写真が多い = there are many photos

So even though English might use there are many..., Japanese often uses this adjective pattern.

It means until the download finishes.

Breakdown:

• ダウンロード = download
• が = subject marker
• 終わる = to finish, to end
• まで = until

So the whole part means:

• ダウンロードが終わるまで = until the download is finished / until the download finishes

Before words like まで, Japanese uses the plain form.

• 終わるまで = until it finishes
• 行くまで = until I go
• 食べるまで = until I eat

This is normal Japanese grammar. Even if the main sentence is polite, the clause before まで usually stays plain:

• ダウンロードが終わるまで待ちます。

So the sentence mixes:

• plain form in the dependent clause: 終わる
• polite form in the main sentence: 待ちます

That is completely natural.

待ちます simply means I wait / I will wait.

In this sentence, it means something like I’ll wait.

• 待ちます = I will wait
• 待っています = I am waiting / I am in the state of waiting

Here, the speaker is stating what they will do, so 待ちます is the most natural choice.

Japanese does not have a separate future tense like English. The non-past form can mean:

• present
• future

So:

• 待ちます can mean wait or will wait

The time is understood from context. In this sentence, it is clearly future-like:

• I will wait until the download finishes

Because here download is being treated as a thing or process that finishes.

• ダウンロードする = to download
• ダウンロードが終わる = the download finishes

So the sentence is not focusing on someone downloading, but on the download process ending.

This is very natural in Japanese.

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

• 待ちます usually implies I wait / I will wait in this kind of sentence

So a fuller English version is:

• Because there are many photos, I’ll wait until the download finishes.

Japanese does this all the time, so omitted subjects are very normal.

No. 写真が多い means there are many photos.

Here, 写真 is plural in meaning because Japanese nouns usually do not change form for singular/plural. So 写真 can mean:

• photo
• photos

In this context, 多い makes it clear that it means photos.

Not exactly. まで means until.

• ダウンロードが終わるまで待ちます = I’ll wait until the download finishes

If you translate it as before, the meaning changes. The Japanese sentence focuses on continuing to wait up to the point when the download ends.

Yes, you can. It would still mean basically the same thing:

• 写真が多いから、ダウンロードが終わるまで待ちます。

The difference is nuance:

• ので = softer, more explanatory
• から = more direct, more conversational

Both are correct here.