Con un aprobado en esta asignatura, me siento más tranquilo.

Not just literally. In this sentence, con introduces the circumstance or condition that makes the speaker feel calmer.

So Con un aprobado en esta asignatura... means something like:

• With a pass in this subject...
• If I get a pass in this subject...
• Once I have a pass in this subject...

This is a very common Spanish use of con: it can mean given that / if I have / with that in place.

Yes, here aprobado is a noun.

From aprobar = to pass / to approve, you get:

• he aprobado = I have passed → past participle use
• un examen aprobado = a passed exam → adjective-like use
• un aprobado = a pass / a passing grade → noun use

In Spain, un aprobado is very common in school and university contexts.

Because un aprobado focuses on the result/grade, not just the action.

Compare:

• Con un aprobado en esta asignatura... = With a pass / a passing mark in this subject...
• Con aprobar esta asignatura... = By passing this subject... or If I pass this subject...

The version with un aprobado sounds more like the speaker is thinking about the minimum satisfactory outcome: even a simple pass would already make them feel calmer.

Not necessarily. In Spain, aprobado usually means a pass, often just the basic passing mark.

So it does not suggest an excellent result. In fact, it can imply:

• I do not need a brilliant grade
• Just passing is enough

That nuance fits well with me siento más tranquilo: the speaker would feel relieved just by securing a pass.

Asignatura means school/university subject or course unit.

For example:

• esta asignatura = this subject / this course
• la asignatura de historia = the history subject/course

A common mistake for English speakers is to confuse it with assignment. It does not mean homework or a specific task. It means the whole subject studied.

Because Spanish often uses en when talking about a grade or result in a subject.

So:

• un aprobado en esta asignatura = a pass in this subject

That is the natural choice here.
You may also hear subject names directly, for example:

• un aprobado en matemáticas

But with asignatura, en is the normal preposition in this structure.

Sentirse means to feel, so me siento is the natural choice when talking about an internal emotional state.

• me siento más tranquilo = I feel calmer / more reassured
• estoy más tranquilo = I am calmer

Both can work, but they are slightly different:

• sentirse emphasizes the speaker’s personal feeling
• estar states the condition more directly

In this sentence, me siento is a very natural match for English I feel.

Because tranquilo agrees with the person who is speaking.

The subject is understood as yo, so the adjective matches the speaker:

• male speaker: me siento más tranquilo
• female speaker: me siento más tranquila

If several people were speaking, it would change accordingly:

• nos sentimos más tranquilos
• nos sentimos más tranquilas

It does not agree with asignatura, even though asignatura is feminine. The adjective describes the speaker, not the subject.

The comparison is implicit.

Spanish often leaves the second part unstated if it is obvious from context. Here it means something like:

• more calm than before
• more calm than I would be without that pass
• more calm than I am now

So más tranquilo does not need an explicit que... if the comparison is already understood.

It can suggest either, depending on context.

By itself, Con un aprobado en esta asignatura, me siento más tranquilo can mean:

• If I get a pass in this subject, I’ll feel more reassured
• Now that I have a pass in this subject, I feel more reassured

The phrase with con is a bit flexible. If you want to make the time reference clearer, Spanish could use:

• Si apruebo esta asignatura, me sentiré más tranquilo → future/conditional idea
• Como ya tengo un aprobado en esta asignatura, me siento más tranquilo → clearly already achieved

It is natural, but not absolutely mandatory in all styles.

The opening phrase Con un aprobado en esta asignatura is an introductory element, and the comma helps mark a pause before the main clause:

• Con un aprobado en esta asignatura, me siento más tranquilo.

Without the comma, the sentence is still understandable:

• Con un aprobado en esta asignatura me siento más tranquilo.

The version with the comma sounds a bit clearer and more carefully punctuated, especially because the opening phrase is not very short.

Yes, a native speaker could say it. It sounds natural, especially in Spain.

Some other common ways to say something similar are:

• Si apruebo esta asignatura, me quedo más tranquilo.
• Con aprobar esta asignatura, me quedo más tranquilo.
• Si saco un aprobado en esta asignatura, me siento más tranquilo.

A useful extra expression is quedarse tranquilo, which often means to feel reassured / to stop worrying. In everyday speech, that can sound very natural too.