tomodati ga siken ni hugoukaku datta node, watasi ha kafe de hanasite nagusamemasita.

Here, 友達が marks friend as the subject of the clause 試験に不合格だった.

• 友達が試験に不合格だった = My friend failed the exam
• が often introduces the subject of a specific event or situation.
• The main topic of the whole sentence is 私 in the second clause: 私はカフェで話して慰めました.

If you used 友達は, it would sound more like you are setting up as for my friend... as a topic, which is possible in some contexts, but が is very natural here for stating what happened to the friend.

不合格 means failure or not passing.
So 試験に不合格だった literally means was unsuccessful in the exam or more naturally failed the exam.

The particle に is used with 合格する / 不合格になる・だ in expressions related to passing or failing:

• 試験に合格する = to pass an exam
• 試験に不合格だ = to have failed an exam / be unsuccessful on an exam

So に marks what the result applies to: the exam.

Both are grammatically correct, but they differ in style.

• 不合格だった = plain style
• 不合格でした = polite style

This sentence mixes a plain-style clause before ので with a polite ending 慰めました, and that is very common in Japanese. The reason clause often stays in plain form even when the main sentence is polite.

So:

• 友達が試験に不合格だったので、私は…慰めました。

is completely natural.

ので means because or since.

In this sentence:

• 友達が試験に不合格だったので = Because my friend failed the exam...

Compared with から:

• ので sounds a bit softer, more explanatory, and slightly more formal
• から can sound more direct or subjective

So both could work:

• 友達が試験に不合格だったので、...
• 友達が試験に不合格だったから、...

But ので fits well in a calm explanation like this.

The comma separates the reason clause from the main clause:

• 友達が試験に不合格だったので、 = because my friend failed the exam,
• 私はカフェで話して慰めました。 = I talked with them at a café and comforted them.

Japanese commas are often used more flexibly than English commas, but here it helps show the pause between reason and main action.

カフェで means at a café.

The particle で marks the place where an action happens:

• カフェで話す = talk at a café
• 学校で勉強する = study at school

By contrast, に is often used for destination, existence, or specific time:

• カフェに行く = go to a café
• カフェにいる = be in a café

Since 話して and 慰めました are actions taking place there, で is correct.

Yes, it connects two actions:

• 話して = talked / spoke, and then...
• 慰めました = comforted

The -て form links actions together. Here it suggests something like:

• I talked with my friend and comforted them
• or I comforted them by talking with them

The exact nuance depends on context, but both ideas are very natural here.

The person being comforted is understood to be the friend.

Japanese often leaves out things that are clear from context. So instead of saying:

• 友達を慰めました

the sentence simply says:

• 慰めました

because the listener can easily understand that the friend is the one being comforted.

If you wanted to make it explicit, you could say:

• 友達を慰めました。

Because 話す does not always need a direct object.

Here, 話して likely means:

• talked
• talked with my friend
• had a conversation

Japanese often omits predictable information. If you wanted to be more specific, you could say:

• 友達と話して慰めました。 = I talked with my friend and comforted them.
• 友達に話して慰めました。 can sound more like talking to the friend.

But in the original sentence, the omitted listener is easy to infer.

Yes. 話す can mean:

• to speak
• to talk
• to tell

In this sentence, the most natural interpretation is talked with or spoke to the friend, because that fits the idea of comforting someone after failing an exam.

So while 話して is broad on its own, context makes the meaning clear.

It can definitely be omitted.

Japanese often drops pronouns when they are obvious from context. So this would also sound natural:

• 友達が試験に不合格だったので、カフェで話して慰めました。

Adding 私は gives a little more contrast or emphasis, as if saying:

• As for me, I comforted them
• or simply making the subject clearer

So 私 is not required, but it is not wrong either.

This is a very common pattern in Japanese.

The subordinate clause is plain:

• 不合格だったので

The main sentence ends politely:

• 慰めました

Japanese usually puts politeness at the end of the sentence, especially in the main clause. So mixing them like this is normal and natural.

A fully plain version would be:

• 友達が試験に不合格だったので、私はカフェで話して慰めた。

A more consistently polite-sounding version might be:

• 友達が試験に不合格だったので、私はカフェで話して慰めました。

The original is already perfectly fine.

Yes, that nuance is possible.

The -て form can simply list actions, but it can also show means or manner. So:

• カフェで話して慰めました

can mean either:

1. I talked with my friend at a café and comforted them
2. I comforted my friend by talking with them at a café

In real use, both ideas overlap, so the sentence sounds natural either way.