Antes de un examen importante me pongo nervioso, pero confío en lo que estudié.

In Spanish, subject pronouns (yo, tú, él, etc.) are often omitted because the verb ending already shows who the subject is.

• me pongo can only mean I become / I get (1st person singular), so yo is optional.
• confío also clearly means I trust / I have confidence.

You can say Yo me pongo nervioso, pero yo confío…, but it sounds heavier and is usually only used for emphasis (e.g., contrasting with someone else).

Literally, me pongo nervioso is “I put myself nervous”, but the natural meaning is “I get nervous” / “I become nervous.”

• poner = to put
• ponerse (reflexive) + adjective = to become / to get (a temporary state)

Examples:

• Me pongo triste = I get sad
• Me pongo contento = I get happy
• Me pongo rojo = I turn red

The me makes poner reflexive (ponerse), which is how Spanish expresses this kind of change in state.

• Me pongo nervioso = I get / I become nervous (focus on the change, the moment the feeling appears).
• Estoy nervioso = I am nervous (focus on the current state, not how you got there).

So in this sentence:

• Antes de un examen importante me pongo nervioso…
  = Right before the exam, I start to feel nervous / I become nervous.

If you said:

• Antes de un examen importante estoy nervioso…
  it would sound more like you are already nervous in that period, but not emphasize the change.

Adjectives in Spanish agree in gender and number with the subject.

• nervioso = masculine singular
• nerviosa = feminine singular
• nerviosos = masculine or mixed group, plural
• nerviosas = feminine plural

The sentence is assuming the speaker is male (or grammatically masculine).
If the speaker is female, she would say:

• Antes de un examen importante me pongo nerviosa…

The verb confiar (to trust / to have confidence) almost always uses the preposition en:

• confiar en alguien = to trust someone
• confiar en algo = to trust something / have confidence in something

So:

• Confío en lo que estudié.
  = I trust / have confidence in what I studied.

Using confío without en would sound incomplete here, and confío a is incorrect in this meaning.

lo que is a common Spanish structure meaning roughly “what” or “that which.”

• lo here is a neuter article (not masculine or feminine).
• que is like that/which.

So:

• lo que estudié ≈ what I studied / the things I studied / the material I studied

Examples:

• No entiendo lo que dices. = I don’t understand what you’re saying.
• Me gusta lo que haces. = I like what you do.

You cannot just say confío en que estudié in this context; that would sound more like “I trust that I studied,” which is odd in Spanish. En lo que estudié clearly refers to the content / material you studied.

estudié is the preterite (simple past), 1st person singular: I studied.

In Latin American Spanish:

• Preterite (estudié) is used for completed actions in the past, even when the time is not specified:

  • Confío en lo que estudié.
    = I trust what I (already) studied.

• Present perfect (he estudiado) is less used than in Spain. It often sounds more like “in my life so far / up to now,” but in many Latin American contexts estudié is preferred instead of he estudiado.

• Imperfect (estudiaba) would mean “I used to study / I was studying”, focusing on an ongoing or repeated action in the past, not just the fact that it got done.

  • Confío en lo que estudiaba would sound strange here in most contexts.

So estudié is the natural choice to talk about specific studying you completed before the exam.

When antes is followed by a noun, Spanish requires de:

• antes de
  • noun
    
    • antes de un examen
    • antes de la cena
    • antes del partido

antes by itself works as an adverb (“before”), but not directly before a noun:

• Correct: Voy a repasar antes del examen.
• Incorrect: Voy a repasar antes el examen.

So you need de before un examen: antes de un examen.

Yes, you can, but the meaning changes slightly.

• antes de un examen importante
  = before an important exam (any such exam, in general; it’s a more general habit).

• antes del examen importante
  = before the important exam (a specific exam that you and the listener know about).

Also:

• del = de + el (before el examen → del examen).

So:

• Talking about your general tendency in life? → antes de un examen importante…
• Talking about a specific exam next week? → antes del examen importante…

In Spanish, most descriptive adjectives normally go after the noun:

• un examen importante = an important exam
• una casa grande = a big house
• un problema serio = a serious problem

You can say un importante examen, but that sounds more formal/literary or emphatic, almost like a really significant exam or a major exam.
In everyday Latin American Spanish, un examen importante is the natural order.

Both can be translated as “but”, but they’re used differently:

• pero = “but / however” (adds a contrast or exception)
• sino = “but rather / but instead” (used after a negation to correct or replace something)

Examples:

• No estoy nervioso, sino tranquilo.
  = I’m not nervous, but rather calm.

In your sentence, the first part is not negative:

• Me pongo nervioso, pero confío en lo que estudié.
  = I get nervous, but I trust what I studied.

Since we’re not negating the first idea, pero is correct, not sino.

examen is masculine:

• el examen = the exam
• un examen = an exam

The plural is exámenes (note the accent shift):

• los exámenes = the exams
• unos exámenes = some exams

So:

• Antes de unos exámenes importantes me pongo nervioso…
  = Before some important exams, I get nervous…

The accents tell you where to put the stress and sometimes distinguish them from other forms:

• confío (I trust)

  • Stress on -fí-: con-FÍ-o
  • Without the accent (confio) the pronunciation rules would place the stress on -fi- anyway, but in many cases with -ío endings, the accent separates the vowels into two syllables: co-nfí-o (three syllables), not con-fio (two).

• estudié (I studied – preterite, yo)

  • Stress on the last syllable: es-tu-di-É
  • Without accent (estudie), it would be es-TU-die, which is a different form (present subjunctive, 3rd person singular or 1st person singular).

So the accents make it clear which tense/form is being used and how to pronounce the word.