O próximo semestre vai ser difícil.

O is the masculine singular definite article, meaning the.

It is used here because semestre is a masculine singular noun: o semestre = the semester.

In Portuguese, articles are used more often than in English, so o próximo semestre is the natural way to say the next semester.

Both word orders can exist in Portuguese, but o próximo semestre is the most natural and common way to say the next semester.

Putting próximo before the noun usually gives the straightforward meaning next/upcoming:

• o próximo semestre = the next semester

After the noun, próximo can sometimes sound a bit more descriptive or formal depending on context:

• o semestre próximo

A learner should generally prefer o próximo semestre.

Próximo usually means next, near, or close, depending on context.

In this sentence, it means next/upcoming:

• o próximo semestre = the next semester

In other contexts, it can mean near:

• a estação mais próxima = the nearest station

So the exact meaning depends on what it is describing.

Vai ser is the construction ir + infinitive, which is a very common way to talk about the future in Portuguese.

So:

• vai ser difícil = is going to be difficult

You could also say:

• será difícil = will be difficult

Both are correct, but vai ser is often more common in everyday spoken language, including in European Portuguese.
Será can sound a bit more formal, more written, or simply more direct.

It is made of:

• vai = third person singular of ir (to go)
• ser = infinitive (to be)

So literally, it is goes to be, but the real meaning is a future construction:

• vai ser = is going to be / will be

Because o próximo semestre is singular, the verb is singular too:

• o semestre vai ser not
• o semestre vão ser

Portuguese has two verbs for to be: ser and estar.

Here, ser is used because difícil describes a characteristic of the semester as a whole, not a temporary physical state or location.

So:

• vai ser difícil = it will be difficult

Using estar here would sound wrong in standard usage.

A useful idea:

• ser = identity, classification, essential description
• estar = state, condition, location

Difficulty is treated here as a quality of the coming semester, so ser is the right choice.

Difícil is an adjective that has the same form for both masculine and feminine singular nouns.

So you can say:

• o semestre é difícil
• a disciplina é difícil

The adjective does not change in the singular.

In the plural, it does change:

• os semestres são difíceis
• as disciplinas são difíceis

So the singular form is simply difícil.

Yes—the written accent shows where the stress falls.

• próximo → stress on the pró
• difícil → stress on the cí

The accent mark is not optional in writing. It is an important part of correct spelling and helps show pronunciation.

For an English speaker, the rough stress pattern is:

• PRÓ-xi-mo
• di-FÍ-cil

In European Portuguese, the unstressed vowels may sound reduced, but the stressed syllable remains important.

Yes, semestre is a normal word and means semester, usually a half-year academic period.

In Portugal, this is commonly used in universities and other educational contexts:

• primeiro semestre = first semester
• segundo semestre = second semester

Depending on the school system, you may also hear other words such as trimestre for a term divided into three parts, but semestre is completely standard.

In standard Portuguese, that sounds incomplete or unnatural in this context.

The normal sentence is:

• O próximo semestre vai ser difícil.

Portuguese usually wants the article here. English often drops the in places where Portuguese keeps it, so this is a common learner mistake.

Yes, it is completely natural in European Portuguese.

Some other natural variations are:

• O próximo semestre vai ser complicado.
• O próximo semestre será difícil.
• O semestre que vem vai ser difícil.

All of these are possible, but O próximo semestre vai ser difícil is clear, standard, and very natural.