Когда брат расстроился из-за плохой оценки, она долго утешала его, а я тоже сочувствовал ему.

Расстроился is the perfective past form, so it presents the event as a single whole: he got upset.

That fits this sentence well, because the idea is:

• first, the brother got upset
• then, she comforted him
• and I sympathized with him too

If you used расстраивался, it would sound more like he was getting upset, he used to get upset, or it would emphasize the process rather than the completed change of state.

Also, notice the -ся ending:

• расстроить кого-то = to upset someone
• расстроиться = to get upset

So брат расстроился means the brother got upset.

Because из-за means because of / due to, and it requires the genitive case.

So:

• dictionary form: плохая оценка = a bad grade
• after из-за: из-за плохой оценки

Both words change to genitive singular:

• плохая → плохой
• оценка → оценки

This is a very common pattern:

• из-за дождя = because of the rain
• из-за работы = because of work
• из-за плохой оценки = because of a bad grade

Because Когда брат расстроился из-за плохой оценки is a subordinate clause introduced by когда (when).

In Russian, subordinate clauses are normally separated by commas:

• Когда он пришёл, мы начали ужинать.
• Если будет дождь, мы останемся дома.

So here:

• Когда брат расстроился из-за плохой оценки, = subordinate clause
• она долго утешала его, а я тоже сочувствовал ему. = main clause

Утешала is imperfective, and that works well because of долго (for a long time). It emphasizes the process of comforting him.

So:

• она долго утешала его = she was comforting him for a long time

If you said утешила, that would sound more like a completed result:

• она утешила его = she comforted him / she succeeded in comforting him

In this sentence, the speaker is focusing on the duration and ongoing action, not just the final result, so утешала is the natural choice.

Because the two verbs require different cases.

1. утешать кого?

This verb takes the accusative.

• утешала его = comforted him

2. сочувствовать кому?

This verb takes the dative.

• сочувствовал ему = sympathized with him

So the forms are different because the grammar of the verbs is different:

• его = accusative/genitive form
• ему = dative form

This is something Russian learners need to memorize with the verb itself:

• любить кого?
• ждать кого?
• помогать кому?
• сочувствовать кому?

That is just how this verb works in Russian: сочувствовать кому-то.

Many Russian verbs take objects in cases other than the accusative. English often uses a preposition instead, but Russian may use a bare case.

So:

• Я сочувствую ему. = I sympathize with him.
• Она сочувствует брату. = She sympathizes with her brother.

This is similar to other verbs such as:

• помогать кому-то = to help someone
• мешать кому-то = to bother/get in someone’s way
• верить кому-то = to believe someone

So it is best to learn сочувствовать together with its case pattern: сочувствовать кому?

Here а marks a switch of subject and a mild contrast in perspective:

• она долго утешала его, а я тоже сочувствовал ему

This means something like:

• she was comforting him, and I also sympathized with him
• or more literally in feeling: she was comforting him, while I also felt sorry for him

So а is not always a strong but. Very often it simply means:

• and meanwhile
• whereas
• as for

Using а here feels natural because the sentence shifts from она to я.

If you used и, it would sound more like simple addition:

• она долго утешала его, и я тоже сочувствовал ему

That is possible, but а is often better when Russian is contrasting or balancing two participants.

Yes. In the Russian past tense, verbs agree in gender and number.

Here we have:

• расстроился — masculine singular
• утешала — feminine singular
• сочувствовал — masculine singular

The interesting part is я сочувствовал. With я, the past-tense verb still shows the speaker’s gender:

• я сочувствовал = the speaker is male
• я сочувствовала = the speaker is female

So this sentence, as written, sounds like it is spoken by a male speaker.

Тоже means also / too.

So:

• я тоже сочувствовал ему = I also sympathized with him / I felt sorry for him too

It shows that the speaker’s reaction is being added to someone else’s reaction. First, she comforted him; I too had a sympathetic reaction.

Russian commonly places тоже before the part it relates to:

• Я тоже пришёл. = I came too.
• Он тоже знает. = He knows too.

In this sentence, тоже naturally attaches to я and the whole idea that follows: I too sympathized with him.

Долго means for a long time or a long time.

So:

• она долго утешала его = she comforted him for a long time

It is an adverb, and it often appears before the verb:

• долго ждал = waited a long time
• долго говорила = spoke for a long time
• долго утешала = comforted for a long time

It helps explain why утешала is imperfective: the speaker is emphasizing duration.

Not completely. Russian word order is more flexible than English word order.

The given sentence has a very natural, neutral order:

• Когда брат расстроился из-за плохой оценки, она долго утешала его, а я тоже сочувствовал ему.

But Russian can move things around for emphasis. For example:

• Когда брат расстроился из-за плохой оценки, его долго утешала она...
• ...а ему я тоже сочувствовал.

Those versions are possible, but they sound more marked and emphasize different words.

The original order is the most straightforward for a learner:

• time clause first
• then the main action
• then the speaker’s added reaction

So the sentence is not rigidly fixed, but the version you were given is the most neutral and natural one.