После того как мы договорились о плане, мне стало спокойнее.

После того как is a fixed conjunction meaning after when it introduces a full clause with a verb.

• После by itself is usually a preposition, so it normally goes before a noun:
  • после встречи = after the meeting
  • после ужина = after dinner

But if you want after + a clause such as after we agreed..., Russian normally uses:

• после того как мы договорились...

So:

• После того как мы договорились о плане... = After we agreed on the plan...

Using just после directly before мы договорились would not be standard Russian.

Because the first part is a subordinate clause:

• После того как мы договорились о плане = subordinate clause
• мне стало спокойнее = main clause

Russian separates subordinate clauses from main clauses with a comma.

So the sentence structure is:

• После того как мы договорились о плане,
  мне стало спокойнее.

That comma is required.

Because the subject is мы = we.

In Russian past tense, the verb agrees with the subject in number, and in the singular also in gender:

• я договорился / договорилась
• мы договорились
• он договорился
• она договорилась
• они договорились

Since мы is plural, the form is договорились.

The -сь is the shortened form of -ся, the reflexive ending.

The verb here is договориться, not just договорить.

That matters because:

• договорить = to finish saying something
• договориться = to come to an agreement

So in this sentence, договорились means we reached an agreement or we agreed.

The reflexive ending here is just part of the dictionary form of the verb. It does not mean a literal reflexive idea like ourselves.

Because договорились is perfective, and it shows a completed result: the agreement was reached.

• договаривались = were negotiating / were in the process of agreeing / used to agree
• договорились = came to an agreement

In this sentence, the important point is that the agreement was completed first, and then the speaker felt calmer. So the perfective verb is the natural choice.

Because the verb договориться takes the pattern:

• договориться о чём-то = to agree on/about something

So:

• о плане = about/on the plan

This is just the government pattern of the verb. English says agree on a plan, but Russian says:

• договориться о плане

The noun план appears here in the prepositional case:

• план → о плане

This is a very common Russian way to talk about changes in a person’s inner state.

• мне стало спокойнее literally means something like it became calmer for me
• natural English meaning: I felt calmer / I became more at ease

Russian often uses this pattern with the dative case for experiences and feelings:

• мне стало лучше = I felt better
• ему стало плохо = he started feeling bad
• нам стало легче = we felt relieved

You can say я стал спокойнее, but it sounds a bit different. It is more directly about I became calmer as a personal state or quality. Мне стало спокойнее is often more natural when talking about emotional relief caused by a situation.

Мне is the dative form of я.

Here the dative marks the person who experiences the state:

• мне стало спокойнее = it became calmer for me
• тебе стало легче = it became easier for you
• ему стало страшно = he became afraid

So мне is not the subject in the usual sense. It is the experiencer.

Стало is the past tense neuter singular form of стать.

In this kind of sentence, стало is used impersonally to show a change of state:

• мне стало спокойно = I became calm / it became calm for me
• мне стало спокойнее = I became calmer / I felt more at ease

So стало means something like became.

The neuter singular form is normal in impersonal constructions like this, even though the sentence is about a person.

Спокойнее is the comparative form of спокойный / спокойно.

Here it means:

• calmer
• more calm
• more at ease

In Russian, comparatives are often used without explicitly saying than before. That comparison is understood from context.

So:

• мне стало спокойнее
  = I felt calmer
  = I felt calmer than before

Not necessarily. It often implies a comparison with an earlier state, even if nothing is stated explicitly.

In this sentence, the meaning is:

• before agreeing on the plan, the speaker was less calm
• after agreeing on it, the speaker felt calmer

So the comparison is understood as compared with before.

Russian does this very often with comparatives:

• стало лучше = it got better
• стало легче = it got easier
• стало спокойнее = it got calmer / I felt calmer

Modern Russian normally does not have a separate everyday verb form equivalent to the English pluperfect had agreed.

Instead, Russian usually shows the sequence through:

• context
• conjunctions like после того как
• aspect

So:

• После того как мы договорились...
  naturally means After we agreed... or After we had agreed...

The order of events is already clear:

1. мы договорились о плане
2. мне стало спокойнее

Yes. Russian word order is flexible.

This sentence can also be said as:

• Мне стало спокойнее после того, как мы договорились о плане.

This still means the same thing.

The main difference is emphasis:

• После того как мы договорились о плане, мне стало спокойнее.
  Focuses first on the earlier event.

• Мне стало спокойнее после того, как мы договорились о плане.
  Focuses first on the result: the speaker felt calmer.

Both are natural.