Хоть день и был длинным, мне стало спокойнее, когда в комнате снова появился порядок.

Here хоть means although / even though.

So:

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

In this kind of sentence, хоть is very close in meaning to хотя.
In modern Russian:

• Хотя день и был длинным... — more neutral/standard
• Хоть день и был длинным... — a little more conversational or expressive

Both are correct.

The и often appears in concessive clauses with хоть or хотя:

• Хотя/хоть X и Y...

It does not mean and here. It helps emphasize the contrast:

• Although the day was long, ...
• Even though the day was long, ...

So и is part of the pattern, not a separate and.

You will often see:

• Хотя он и устал, он продолжал работать.
• Хоть было и поздно, мы не уходили.

Russian can sometimes omit this и, but with хоть / хотя it is very common.

This is because длинным is the instrumental case of the adjective длинный.

In Russian, when an adjective is used as part of the predicate after быть in the past or future, the instrumental is often used:

• День был длинным.
• Разговор будет коротким.

So:

• длинный = nominative
• длинным = instrumental singular masculine/neuter

You may also hear день был длинный, especially in speech, but был длинным is a very standard and natural pattern.

Both are possible in Russian, but they are slightly different in structure and feel.

In this sentence:

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

This is an impersonal construction often used for feelings and states:

• мне стало легче = I felt better / it became easier for me
• ему стало грустно = he became sad / he felt sad
• нам стало весело = we started feeling cheerful

Compare:

• я стал спокойнее = I became calmer
  This focuses more directly on I as the subject.
• мне стало спокойнее = I felt calmer / it became calmer for me
  This is very natural when describing an internal emotional change.

So the sentence uses the more idiomatic “state changed for me” structure.

Стало is the past tense neuter singular form of стать.

Here it is used in an impersonal construction:

• мне стало спокойнее

There is no normal grammatical subject like я or что-то. Russian often uses neuter past tense singular in this kind of sentence about states, feelings, weather, and general situations.

Examples:

• Мне стало холодно. = I got cold.
• Ей стало плохо. = She felt ill.
• Стало темно. = It got dark.

So стало here means became in an impersonal sense.

Спокойнее is the comparative of спокойный / спокойно.

It means:

• calmer
• more calm
• more at ease

So мне стало спокойнее literally means:

• it became calmer for me
• idiomatically: I felt calmer

The comparative is used because the sentence describes a change from one state to another.

Compare:

• Мне спокойно. = I am calm.
• Мне стало спокойнее. = I felt calmer / I became calmer.

Because в комнате means in the room, expressing location, and after в with location Russian uses the prepositional case.

• комната = room
• в комнате = in the room

Compare location vs movement:

• в комнате = in the room (where?)
• в комнату = into the room (to where?)

Here the sentence is talking about where order appeared, so location is needed:

• когда в комнате снова появился порядок
  = when order appeared in the room again

Появиться means to appear / to show up / to come into being.

So:

• в комнате снова появился порядок literally means
• order appeared in the room again

In natural English, that might be translated more freely as:

• the room became tidy again
• order was restored in the room
• the room was orderly again

Russian often uses an abstract noun like порядок (order, tidiness) with появился to express the return of organization or neatness.

Because появился emphasizes that the order came back or appeared again after not being there.

Compare:

• В комнате был порядок. = The room was tidy.
  This just describes the state.
• В комнате снова появился порядок. = Order appeared in the room again.
  This emphasizes a change or restoration.

That fits well with мне стало спокойнее: the speaker felt calmer when the room returned to an orderly state.

Снова means again.

So:

• в комнате снова появился порядок = order appeared in the room again

This suggests that earlier the room had order, then it was lost, and then it returned.

Without снова, the sentence would simply say that order appeared.
With снова, it clearly means returned.

Yes, Russian word order is flexible, but the chosen order sounds natural and gives good information flow.

The sentence is:

• Хоть день и был длинным, мне стало спокойнее, когда в комнате снова появился порядок.

A rough structure is:

1. concessive clause: Although the day was long
2. main clause: I felt calmer
3. time clause: when order returned to the room

Russian could rearrange parts for emphasis, for example:

• Мне стало спокойнее, когда в комнате снова появился порядок, хоть день и был длинным.

But the original order is smoother and more natural in most contexts.

Inside the last clause, you could also say:

• когда порядок снова появился в комнате

But когда в комнате снова появился порядок sounds very natural because it first sets the scene (in the room) and then presents what happened there (order appeared again).

Because the sentence contains subordinate clauses.

1. Хоть день и был длинным is a subordinate concessive clause.
2. когда в комнате снова появился порядок is a subordinate time clause.

So the commas separate:

• the although clause from the main clause
• the when clause from the main clause

That is why the sentence is written:

• Хоть день и был длинным, мне стало спокойнее, когда в комнате снова появился порядок.

A fairly literal translation would be:

• Although the day was long, it became calmer for me when order appeared in the room again.

A more natural English translation would be:

• Although the day was long, I felt calmer when the room was tidy again.
• Even though the day was long, I felt calmer when order returned to the room.

The Russian wording is slightly more abstract than typical English, especially with:

• мне стало спокойнее
• появился порядок

But it is very natural Russian.