dousite sonnani osokatta ndesu ka? douro ga zyuutaisite ita ndesu ka?

Because the speaker is asking about a completed situation: the person was late. In Japanese, when you talk about someone arriving late and ask about that past event, 遅かった is natural.

• 遅い = is late / slow
• 遅かった = was late

So どうしてそんなに遅かったんですか means Why were you so late?

そんなに means that much, to that extent, or more naturally here, so.

So:

• そんなに遅かった = were that late / were so late

It adds emphasis to the degree of lateness. The speaker is not just asking why the person was late, but why they were so late.

んですか adds an explanatory or reason-seeking tone. It often sounds like:

• How come ...?
• What was the reason ...?
• Was it that ...?

So:

• どうしてそんなに遅かったんですか？ = How come you were so late?
• 道路が渋滞していたんですか？ = Was it that the road was congested?

Without んですか, the questions would sound more plain and less connected to an explanation.

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

In どうしてそんなに遅かったんですか？, the speaker is clearly talking to the other person, so あなたは is unnecessary.

A literal full version could be something like:

• あなたはどうしてそんなに遅かったんですか？

But in natural Japanese, that would usually sound less natural unless the subject needs emphasis.

Here, が marks the road as the thing being identified as the possible cause.

The speaker is asking whether the road being congested is the explanation. So が fits well because it points to the specific thing involved in the situation.

• 道路が渋滞していたんですか？ = Was it the road that was congested?

If you used は, it would sound more like you were setting the road up as a topic for comparison or contrast, which is not the main point here.

It comes from the verb 渋滞する, which means to be congested or to be backed up.

Breakdown:

• 渋滞する = to be congested / to have a traffic jam
• 渋滞している = is congested / is in a state of congestion
• 渋滞していた = was congested

So 道路が渋滞していた means the road was congested.

Because 渋滞していた describes an ongoing state: the road was in a condition of congestion.

• 渋滞した can sound more like became congested
• 渋滞していた means was congested / had been congested

In this sentence, the speaker is asking about the condition that may have caused the delay, so 渋滞していた is the more natural choice.

Yes, it is understandable and natural enough, but in everyday conversation, people also often say things like:

• 道が混んでいたんですか？ = Was the road crowded/heavy with traffic?
• 渋滞していたんですか？ = Was there a traffic jam?

So 道路が渋滞していた is fine, but it can sound slightly more explicit or formal than casual spoken Japanese.

Both mean why.

• どうして is very common in everyday conversation
• なぜ can sound a bit more formal, direct, or written

In this sentence, どうして sounds natural and conversational.

So:

• どうしてそんなに遅かったんですか？ = natural spoken Japanese
• なぜそんなに遅かったんですか？ = also correct, but slightly more formal

Yes. The second sentence, 道路が渋滞していたんですか？, sounds like the speaker is offering a possible explanation and asking if it is correct.

So the flow is:

1. Why were you so late?
2. Was the road congested?

That makes the second question feel like Was that the reason?

The dictionary form is 遅い.

This is an i-adjective, and its past form is made like this:

• 遅い → 遅かった

That is why 遅かったんですか is built from the adjective 遅い.

Yes. んですか is basically the conversational form of のですか.

So these are equivalent in meaning:

• どうしてそんなに遅かったんですか？
• どうしてそんなに遅かったのですか？

And:

• 道路が渋滞していたんですか？
• 道路が渋滞していたのですか？

The んです version is more common in speech, while のです sounds more formal or stiff.