Хоть день и был длинным, мне стало спокойнее, когда мы спокойно обсудили всё друг с другом.

Хоть ... и is a very common concessive pattern, meaning although / even though.

So:

• Хоть день и был длинным = Although the day was long

The и does not mean a simple and here. In this pattern, it helps mark the concessive idea more clearly. You will often see:

• хоть ... и ...
• хотя ... и ...

Both are common, and both mean although / even though.

Compare:

• Хоть день и был длинным, ...
• Хотя день и был длинным, ...

Both are natural.

Here длинным is in the instrumental case because it is part of the predicate after был.

With быть in the past or future, Russian often uses the instrumental for the predicate noun or adjective:

• Он был уставшим.
• День был длинным.

This is very common and natural.

You may also sometimes hear nominative in similar sentences, especially in more conversational styles:

• День был длинный.

But был длинным is standard and very natural here.

A useful learner rule is:

• after был / будет / стал / оказался, predicate adjectives are often in the instrumental

Because Russian often expresses feelings and internal states with a dative experiencer plus an impersonal construction.

• мне стало спокойнее literally means something like it became calmer to me
• natural English meaning: I felt calmer / I became calmer

This structure is very common in Russian for physical or emotional states:

• Мне стало грустно. = I became sad.
• Мне стало легче. = I felt better / It became easier for me.
• Мне стало страшно. = I got scared.

You can say я стал спокойнее, but that sounds more like a statement about me as a person / my manner / my overall state.
Here, мне стало спокойнее focuses more on the speaker’s inner feeling in that moment.

Спокойнее is the comparative form of спокойный / спокойно.

In this sentence it means calmer / more at ease.

It works here as part of a state expression:

• мне стало спокойнее

So it is not describing a noun directly. It is describing the speaker’s state.

Compare:

• Я стал спокойнее. = I became calmer.
• Мне стало спокойнее. = I felt calmer / I became calmer inside.

Russian comparatives like лучше, хуже, легче, спокойнее are very often used this way.

They are related, but they do different jobs.

• спокойнее = more calmly / calmer in a comparative sense
• спокойно = calmly in a basic, non-comparative sense

In the sentence:

• мне стало спокойнее = I felt calmer
• мы спокойно обсудили = we discussed calmly

So:

• спокойнее describes a change of state
• спокойно describes how the discussion happened

Because the sentence refers to a completed discussion.

• обсудили is perfective
• it presents the action as a whole, with a result: the discussion happened and was finished

This fits well with когда here:

• когда мы спокойно обсудили всё друг с другом
  = when we had calmly discussed everything / when we calmly talked everything through

If you used the imperfective обсуждали, the focus would be more on the process:

• когда мы спокойно обсуждали... = when we were calmly discussing...

But here the important point is that after the discussion was completed, the speaker felt calmer.

Because всё means everything, while все means everyone or all (people/things) depending on context.

Here the meaning is:

• обсудили всё = discussed everything

Compare:

• всё = everything
• все = everyone / all

Examples:

• Мы обсудили всё. = We discussed everything.
• Все пришли. = Everyone came.

The two words are easy to mix up, so this is a very common learner question.

Друг с другом means with each other / with one another.

So:

• мы обсудили всё друг с другом = we discussed everything with each other

This expression is very common when the action is mutual.

It changes by case depending on the preposition or the verb:

• друг друга = each other (accusative/genitive-type form)
• друг другу = to each other
• друг с другом = with each other
• друг о друге = about each other

Here the preposition с requires the instrumental, so you get:

• друг с другом

It is not strictly necessary, but it adds emphasis and clarity.

• Мы обсудили всё. = We discussed everything.
• Мы обсудили всё друг с другом. = We discussed everything with each other.

Since обсудить already usually involves more than one side, the sentence works without it.
But друг с другом stresses that this was a two-way conversation between the people involved, not just one person explaining things to the others.

So it is natural and stylistically useful, even if not absolutely required.

Когда means when, and here it introduces the event after which the speaker felt calmer.

The structure is:

• first clause: мне стало спокойнее
• subordinate clause with когда: когда мы спокойно обсудили всё друг с другом

This means the calming happened when / after the discussion took place.

Because обсудили is perfective, the clause suggests a completed event. So the idea is close to:

• I felt calmer once we had calmly discussed everything with each other

So когда here is not just a simple time label; it links the emotional change to the completed conversation.

Yes, Russian word order is flexible, and this sentence is arranged in a natural, information-flow way.

Current order:

• Хоть день и был длинным, мне стало спокойнее, когда мы спокойно обсудили всё друг с другом.

This starts with the concessive background:

• Although the day was long...

Then it gives the main result:

• I felt calmer...

Then it explains when/why:

• when we calmly discussed everything with each other

You could rearrange parts, for example:

• Мне стало спокойнее, хоть день и был длинным, когда мы спокойно обсудили всё друг с другом.

But that is less smooth.

The original order sounds natural because it moves from:

1. background circumstance,
2. main emotional result,
3. specific triggering event.

Yes. In this sentence, хоть and хотя are both possible.

• Хоть день и был длинным...
• Хотя день и был длинным...

Both mean although the day was long...

In many contexts:

• хотя is a bit more neutral and standard
• хоть can feel a little more conversational or compact

But both are completely normal here.

It is a very useful Russian pattern for changes in physical or emotional state:

• мне стало лучше = I felt better
• мне стало хуже = I felt worse
• мне стало легче = I felt relieved / It became easier for me
• мне стало спокойнее = I felt calmer

The structure is:

• dative person
  • стало
    • state/comparative word

This is one of the most natural ways in Russian to talk about how someone begins to feel. It is worth learning as a whole pattern, not just as separate words.