Когда учитель всё объяснил, мне стало спокойнее.

Because Когда учитель всё объяснил is a subordinate clause introduced by когда (when). In Russian, subordinate clauses are separated by a comma.

So the sentence is structured like this:

• Когда учитель всё объяснил = When the teacher explained everything / had explained everything
• мне стало спокойнее = I felt calmer / it became calmer for me

Russian uses this comma even when English sometimes feels less strict about it.

Here когда means when and introduces a time clause.

In this sentence, it means:

• Когда учитель всё объяснил... = When the teacher had explained everything...

It refers to a specific moment or situation, not a general whenever meaning.

Depending on context, когда can mean either:

• when (at what time / after the moment that)
• whenever (in repeated situations)

Here it is clearly a one-time past event.

Объяснил is the perfective past form of объяснить, meaning the action was completed.

So:

• объяснил = explained / finished explaining
• объяснял = was explaining / explained with focus on the process, not necessarily completion

In this sentence, the idea is that the teacher finished explaining everything, and after that, the speaker felt calmer. That is why perfective объяснил fits naturally.

Compare:

• Когда учитель всё объяснил, мне стало спокойнее.
  When the teacher finished explaining everything, I felt calmer.

• Когда учитель всё объяснял, я слушал.
  While the teacher was explaining everything, I was listening.

That is the standard marker of the past tense in Russian.

Past tense forms in Russian also show gender and number:

• объяснил = masculine singular
• объяснила = feminine singular
• объяснило = neuter singular
• объяснили = plural

So объяснил tells you that the subject is singular and grammatically masculine. Since the subject is учитель, that matches.

Учитель means teacher and is a masculine noun. Because the verb is объяснил, the sentence specifically refers to a male teacher.

If it were a female teacher, you would usually say:

• Когда учительница всё объяснила, мне стало спокойнее.

So the sentence as written points to a male teacher.

Because всё and все are different words.

• всё = everything
• все = everyone / all (people or things in plural)

Here the meaning is everything, so it must be всё:

• учитель всё объяснил = the teacher explained everything

In everyday writing, Russians sometimes omit the dots and write все, but in careful writing the difference matters.

Because Russian often uses the dative case to express a person's state, feeling, or experience.

So:

• мне = to me / for me
• literally, it became calmer to me
• natural English: I felt calmer

This is a very common Russian pattern:

• мне холодно = I am cold
• мне грустно = I am sad
• мне стало легче = I felt better / it became easier for me
• мне стало спокойнее = I felt calmer

Russian often avoids making I the grammatical subject in these expressions.

This is an impersonal construction.

The verb стало is the past tense neuter singular form of стать. In sentences like this, Russian often uses neuter singular by default when there is no normal subject.

So:

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

This same pattern appears in many common expressions:

• мне стало плохо = I started feeling bad
• ему стало страшно = he became scared
• нам стало ясно = it became clear to us

The neuter form does not mean some real neuter object is doing the action; it is just the normal shape of the verb in impersonal sentences.

Спокойнее is the comparative form of спокойный / спокойно and means calmer or more at ease.

In this sentence:

• мне стало спокойнее = I felt calmer
• more literally: it became calmer for me

Russian comparatives are often used without a word meaning more:

• спокойный = calm
• спокойнее = calmer

This form works very naturally in impersonal feeling expressions.

Both are possible in Russian, but they are not exactly the same in tone.

• мне стало спокойнее focuses on the internal feeling/state
• я стал спокойнее sounds more like I became a calmer person or I grew calmer as a quality of myself

In this context, after the teacher explained everything, the speaker's emotional state changed. Russian very naturally expresses that with:

• мне стало спокойнее

So this version is more idiomatic for a temporary feeling of relief.

Yes, that is possible. Russian word order is fairly flexible.

Both of these are correct:

• Когда учитель всё объяснил, мне стало спокойнее.
• Когда учитель объяснил всё, мне стало спокойнее.

The difference is mostly about emphasis and rhythm, not basic meaning.

• всё объяснил can slightly emphasize everything
• объяснил всё is also very natural and neutral

Russian word order often changes depending on what the speaker wants to highlight.

In English, the most natural translation is usually I felt calmer.

A more literal translation would be:

• it became calmer for me

And a slightly more direct English version could be:

• I became calmer

But in normal English, I felt calmer often sounds best because the Russian phrase is about the speaker's emotional state, not a dramatic change in personality.

Yes. Russian comparatives often leave the standard of comparison unstated when it is obvious from context.

So спокойнее here means something like:

• calmer than before
• calmer than I was a moment ago
• calmer after the explanation than before it

Russian does not need to spell that out.

If you want to state the comparison explicitly, Russian can do that too:

• мне стало спокойнее, чем раньше = I felt calmer than before

Yes. With a perfective verb like объяснил, когда often has the sense of when/once/after in context.

So this sentence can feel like:

• When the teacher had explained everything, I felt calmer
• After the teacher explained everything, I felt calmer
• Once the teacher explained everything, I felt calmer

The exact English wording depends on style, but the Russian idea is that the explanation was completed first, and then the speaker's state changed.

It is a very common Russian pattern for talking about a change in state.

Pattern:

• [dative person] + стало + state word

Examples:

• мне стало легче = I felt better
• ей стало грустно = she became sad
• нам стало интереснее = it became more interesting for us
• ему стало спокойнее = he felt calmer

This is one of the most useful patterns for learners because it appears constantly in natural Russian.

Grammatically yes, but the meaning changes.

• Когда учитель всё объяснил... = when the teacher finished explaining everything
• Когда учитель всё объяснял... = while the teacher was explaining everything

With объяснял, the focus is on the process. That could mean the speaker started feeling calmer during the explanation, not after it was fully completed.

So if the intended meaning is that the full explanation reassured the speaker, объяснил is the better choice.