Learn Hardcore Spanish (Spain): Mi hermano universitario está feliz porque su contrato de prácticas se convertirá en contrato fijo. - My university‑student brother is happy because his internship contract will turn into a permanent contract.

In this sentence, the happiness is clearly a reaction to the news about the contract, so está feliz is the natural choice. You could say es feliz but it would sound like you are describing his general outlook on life, not his reaction to the contract.

What does universitario add in mi hermano universitario? Is this the usual way to say “my brother who is at university”?

Universitario here is an adjective that means “who goes to university / university student.”

So mi hermano universitario means “my brother who is a university student.”
It is a normal, concise way to say this in Spain.

You could also say:

mi hermano que está en la universidad
mi hermano que estudia en la universidad
mi hermano estudiante universitario

All are correct. Mi hermano universitario is just shorter and sounds quite natural in European Spanish.

Why is it contrato de prácticas and not contrato de práctica?

In Spain, las prácticas (always in the plural) is the usual expression for an internship / work placement.

hacer prácticas = to do an internship
contrato de prácticas = an internship contract

It is not about the number of internships; prácticas is simply the fixed way to refer to this kind of training period. Contrato de práctica would sound incorrect or at least very odd to native speakers.

Why is se convertirá reflexive here? Could we just say convertirá?

Spanish distinguishes between:

convertirse en (reflexive): the subject changes its own nature/state
- El contrato de prácticas se convertirá en contrato fijo.
  The internship contract itself becomes a permanent contract.
convertir algo en algo (non‑reflexive): someone/something changes something else
- La empresa convertirá su contrato de prácticas en un contrato fijo.
  The company will turn his internship contract into a permanent one.

In your sentence, the focus is on the contract itself changing, so the reflexive se convertirá is the natural choice.

Why do we say convertirse en and not convertirse a or another preposition?

In Spanish, when something turns into / becomes something else, the standard structure is:

convertirse en + noun

Examples:

El agua se convierte en hielo.
La amistad se convirtió en amor.
Su contrato de prácticas se convertirá en contrato fijo.

You do not say convertirse a contrato fijo or convertirse en a contrato fijo. The correct preposition with convertirse in this sense is en.

Could we also say va a convertirse en contrato fijo instead of se convertirá en contrato fijo? What’s the difference?

Yes, both are grammatically correct and quite natural:

se convertirá en contrato fijo – simple future
va a convertirse en contrato fijo – ir a + infinitive (periphrastic future)

Differences in nuance:

Simple future (se convertirá) often sounds a bit more formal or neutral; common in written Spanish and in statements or predictions.
Ir a + infinitive (va a convertirse) can sound a bit more conversational and can suggest a more “planned” or “near” future, though the difference is subtle.

In this sentence, either form works well; the choice is mostly stylistic.

Why is there no article in en contrato fijo? Why not en un contrato fijo?

After verbs like convertirse en, hacerse, llegar a ser, Spanish often omits the article when talking about a new status, role or category in a general sense:

Se convirtió en director.
Llegó a ser presidente.
Su contrato se convertirá en contrato fijo.

Here, contrato fijo is a type/category of contract, not a specific one, so omitting the article is very natural.

You could say en un contrato fijo, and it would still be correct. That version tends to sound a bit more concrete or specific (as in “into a permanent contract” as a particular instance), but the difference is small.

Why does the adjective fijo come after contrato and not before it?

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

contrato fijo – permanent contract
casa grande
coche nuevo

Putting fijo before the noun (fijo contrato) would sound ungrammatical here.

Some adjectives can go before or after the noun with a change of meaning or nuance, but fijo with contrato is a fixed, conventional combination, and the natural order is contrato fijo.

Is está feliz the most natural way to say “is happy” here, or would está contento be better?

Both are correct, but there is a subtle nuance:

está feliz – sounds a bit stronger or more intense, like “he is (really) happy / delighted.”
está contento – a bit more everyday and slightly softer: “he is pleased / glad / happy.”

In Spain, está contento is extremely common for situations like this.
Mi hermano universitario está contento porque… might be more colloquial and typical in daily speech, but está feliz is perfectly natural too, just a bit more emphatic.

How would the sentence change if we were talking about a sister instead of a brother?

You need to change the words that agree in gender:

Mi hermano universitario está feliz porque su contrato de prácticas se convertirá en contrato fijo.
→ masculine (brother)

For a sister:

Mi hermana universitaria está feliz porque su contrato de prácticas se convertirá en contrato fijo.

Changes:

hermano → hermana
universitario → universitaria

If it were plural (brothers/siblings), you would also change number:

Mis hermanos universitarios están felices porque…

What is the difference between porque in this sentence and por qué / porqué / por que?

Spanish has four different forms:

porque (one word, no accent) – means “because”
- Mi hermano está feliz porque su contrato…
  He is happy because his contract…
por qué (two words, accent on qué) – used in questions (direct or indirect)
- ¿Por qué está feliz tu hermano?
- No sé por qué está feliz.
porqué (one word, accent) – a noun meaning “reason / cause”
- No entiendo el porqué de su felicidad.
  I don’t understand the reason for his happiness.
por que (two words, no accent) – less common; appears in specific grammatical combinations (e.g. after certain prepositions or verbs), often replaceable by por el que / por la que, etc.

In your sentence, we need porque (because), since it introduces the reason for his happiness.