Хотя день и был длинным, вечером мне стало спокойнее, когда я полил кактус и выключил телефон.

This и is a particle that often appears in the pattern хотя ... и ....

It does not usually need a separate translation into English. It adds a concessive feel, something like:

• although the day was long
• even though the day was long

So:

• Хотя день был длинным = correct
• Хотя день и был длинным = also correct, and a bit more idiomatic or emphatic in this kind of sentence

English speakers often expect и to mean only and, but here it does not mean and.

Because after быть in the past tense, a predicate adjective in its full form can appear in the instrumental.

So:

• день был длинным = the day was long

This is a normal pattern in Russian. You may also see nominative in some cases:

• день был длинный

Both are possible, but the instrumental often sounds more like a description or characterization of the situation/state. In this sentence, длинным is natural and standard.

Russian often uses the instrumental case of certain time words to mean in/during that time.

So:

• утром = in the morning
• днём = in the daytime
• вечером = in the evening
• ночью = at night

So вечером by itself already means in the evening. No preposition is needed.

Russian very often expresses feelings and internal states with an impersonal construction:

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

This is extremely common and idiomatic.

Compare:

• я стал спокойнее = I became calmer
  
  • focuses more on me as the subject
• мне стало спокойнее = it became calmer for me / I felt calmer
  
  • focuses more on the change in inner state

Both are possible in many contexts, but мне стало спокойнее often sounds more natural for emotions, comfort, relief, and similar states.

Because this is an impersonal sentence: there is no grammatical subject like я controlling the verb.

In Russian, impersonal past-tense constructions often use the neuter singular form:

• стало
• было
• сделалось

So:

• мне стало спокойнее = literally it became calmer to me

The verb is neuter singular because the sentence is built without a regular subject.

Спокойнее is the comparative form of спокойный / спокойно.

It means:

• calmer
• more calm
• more at ease

In this sentence it functions as a predicative word of state: it describes how the speaker felt.

Russian often uses simple comparative forms like this:

• лучше = better
• хуже = worse
• легче = easier
• спокойнее = calmer

So мне стало спокойнее is a very natural Russian way to say I felt calmer.

Because both actions are presented as completed, one-time actions:

• полил кактус = watered the cactus
• выключил телефон = turned off the phone

The perfective aspect fits the idea that these actions were finished, and then the speaker felt calmer.

Compare:

• полил / выключил = completed actions, result matters
• поливал / выключал = process, repeated action, or background action

Here the result is important: after doing those things, the speaker felt better.

They are in the accusative case as direct objects, but for inanimate masculine singular nouns, the accusative has the same form as the nominative.

So:

• nominative: кактус
• accusative: кактус

and:

• nominative: телефон
• accusative: телефон

That is why you get:

• полил кактус
• выключил телефон

If the noun were animate masculine, the accusative would usually look like the genitive instead.

Here когда can naturally mean when, once, or after, depending on how you express it in English.

Because the verbs in that clause are perfective:

• когда я полил кактус и выключил телефон

the sense is close to:

• when I watered the cactus and turned off the phone
• once I had watered the cactus and turned off the phone
• after I watered the cactus and turned off the phone

Russian когда is often broader than English learners first expect.

Because Russian uses commas to separate subordinate clauses.

Here there are two subordinate clauses:

• Хотя день и был длинным, ...
  This is a concessive clause introduced by хотя.

• ..., когда я полил кактус и выключил телефон.
  This is a time clause introduced by когда.

So the commas are correct and expected.

There is no comma before the и in полил кактус и выключил телефон because those are just two coordinated verbs with the same subject я inside the same clause.

No. Russian word order is flexible, and changing it often changes emphasis, not basic meaning.

This version:

• Хотя день и был длинным, вечером мне стало спокойнее, когда я полил кактус и выключил телефон.

puts the ideas in this order:

1. concession/background: although the day was long
2. time frame: in the evening
3. experiencer/state: I felt calmer
4. trigger/time clause: when I watered the cactus and turned off the phone

Other word orders are possible, for example:

• Хотя день и был длинным, мне вечером стало спокойнее...
• Вечером мне стало спокойнее, хотя день и был длинным...

The meaning stays very similar, but the focus shifts.

Only if the meaning changed.

In the actual sentence, perfective verbs are used because the speaker means completed actions:

• полил
• выключил

If you used imperfective forms:

• поливал
• выключал

it would suggest something more like:

• the process of watering / turning off
• repeated or habitual action
• background description rather than a completed result

So for this sentence, perfective is the natural choice because the emotional change comes after those actions are done.