Se a matrícula estiver completa, eu passo a preocupar-me menos com o início do curso.

Because se here introduces a future condition: if the enrolment/registration is complete at some later point.

In Portuguese, when se means if and refers to something uncertain in the future, the verb normally goes in the future subjunctive.
That is why you get:

• Se a matrícula estiver completa... = If the registration is complete / once the registration is complete...

Here, estiver is the future subjunctive of estar.

Using está here would sound non-standard in careful Portuguese for this kind of future condition.

Estiver is the future subjunctive, 3rd person singular, of estar.

The subject is a matrícula, which is grammatically singular, so the verb is singular too.

A few examples of the same pattern:

• Se eu estiver pronto... = If I am ready...
• Se ele estiver em casa... = If he is at home...
• Se a matrícula estiver completa... = If the registration is complete...

This tense is very common in Portuguese after words like se, quando, logo que, assim que, when the meaning is future.

Here, matrícula means enrolment / registration.

In educational contexts in Portugal, fazer a matrícula means to enrol or to register for a course, school, or programme.

So a matrícula estiver completa most naturally means something like:

• the enrolment/registration has been fully completed
• or the registration process is complete

It does not mean number plate here, even though matrícula can have that meaning in other contexts.

Passar a + infinitive is a very common structure meaning:

• to start to
• to begin to
• to come to

So:

• eu passo a preocupar-me menos = I start worrying less / I become less worried

It suggests a change of state: before, the speaker was more worried; after the registration is complete, that worry decreases.

Other examples:

• Passei a estudar mais. = I started studying more.
• Ela passou a trabalhar em casa. = She started working from home.

This is a very useful European Portuguese point.

In European Portuguese, object/reflexive pronouns are often attached after the infinitive:

• preocupar-me

So passo a preocupar-me is the normal European Portuguese pattern.

In Brazilian Portuguese, you will very often hear:

• passo a me preocupar

That sounds much more Brazilian.
For Portugal, preocupar-me is the more standard and expected form.

Because the expression is passar a + infinitive.

So the structure is:

• passo a preocupar-me
• literally: I pass to worry myself
• naturally: I start to worry less

That a belongs to the verbal expression passar a, not to preocupar by itself.

Compare:

• começar a estudar = to start studying
• voltar a tentar = to try again
• passar a preocupar-me menos = to start worrying less

Because the verb is preocupar-se com = to worry about / to be concerned about.

So the full idea is:

• preocupar-me com o início do curso = to worry about the start of the course

Then menos simply reduces the degree:

• preocupar-me menos com... = to worry less about...

So the structure is:

• preocupar-se com alguma coisa
• worry about something

Examples:

• Preocupo-me com os exames. = I worry about the exams.
• Preocupo-me menos com isso. = I worry less about that.

Not really. Portuguese often drops subject pronouns because the verb ending already shows the person.

So both are possible:

• Se a matrícula estiver completa, eu passo a preocupar-me menos...
• Se a matrícula estiver completa, passo a preocupar-me menos...

The version with eu may add a little emphasis, contrast, or clarity.
Without eu, the sentence still sounds perfectly natural.

Because the sentence starts with a conditional subordinate clause:

• Se a matrícula estiver completa

and then gives the main clause:

• eu passo a preocupar-me menos com o início do curso

In Portuguese, when this kind of clause comes first, it is very common to separate it with a comma.

So the structure is:

• If X, then Y

That is why the comma is there.

Because this refers to a specific course and its beginning.

• o início = the beginning / the start
• do curso = of the course (de + o = do)

So:

• o início do curso = the start of the course

Using the definite article is very normal in Portuguese, often more than in English.

Compare:

• o fim do ano = the end of the year
• o início da aula = the start of the lesson
• o início do curso = the start of the course

Not exactly.

• preocupo-me menos = I worry less
• passo a preocupar-me menos = I start to worry less / I come to worry less

The version with passo a highlights a change. It suggests that once the registration is complete, the speaker's level of concern changes from that point onward.

So:

• preocupo-me menos = simple description
• passo a preocupar-me menos = description of a new phase or transition

That extra nuance is important here.

In this sentence, completa is an adjective describing the state of a matrícula.

So in English, depending on context, it may correspond to either:

• complete
• completed

Portuguese often uses a simple adjective where English might choose either an adjective or a past participle.

So:

• a matrícula estiver completa

can suggest:

• the registration is complete
• the enrolment has been completed

The exact English wording depends on the wider context, but the Portuguese itself is fully natural.