Learn Hardcore Portuguese (Portugal): Ela acha que está a ser preguiçosa quando não estuda à noite. - She thinks she is being lazy when she doesn’t study at night.

achar (in this sense) is to think / to feel / to have the impression that. It’s often more subjective and more informal, like English I think / I feel like / I guess:
- Ela acha que está a ser preguiçosa.
  She thinks / feels (in her opinion) that she is being lazy.
pensar is more literally to think, and can sound a bit more neutral, logical, or deliberate:
- Ela pensa que está a ser preguiçosa. – also correct, but can feel slightly more “rational” or less casual depending on context.

In everyday European Portuguese speech, achar is extremely common for expressing opinions and impressions. Pensar is also correct, but achar is usually the more natural choice in a sentence like this.

What exactly does está a ser mean? Why not just é?

Está a ser is the present continuous (progressive) of ser: literally “is being”.

Ela acha que está a ser preguiçosa.
She thinks she is being lazy (right now / in this situation).

The nuance:

está a ser preguiçosa = she is acting lazy, behaving lazily in a certain period or context.
é preguiçosa = she is a lazy person (a general, permanent characteristic).

So if she only feels lazy when she doesn’t study at night (but is not necessarily a lazy person overall), está a ser preguiçosa is the natural choice.

Why is it está a ser and not está sendo, like in Brazilian Portuguese?

This sentence is in European Portuguese. The normal way to form the present continuous in European Portuguese is:

estar + a + infinitive

Examples:

está a estudar – is studying
estão a comer – are eating
estou a ler – I am reading
está a ser preguiçosa – is being lazy

In Brazilian Portuguese, the usual pattern is:

estar + gerúndio

So a Brazilian would normally say:

Ela acha que está sendo preguiçosa quando não estuda à noite.

In European Portuguese, está sendo is understood but sounds Brazilian or marked; está a ser is the standard form.

Why is preguiçosa in the feminine form? Could it be preguiçoso?

Adjectives in Portuguese agree in gender and number with the noun or pronoun they describe.

Ela (she) is feminine singular.
So the adjective must also be feminine singular: preguiçosa.

Forms of this adjective:

Masculine singular: preguiçoso
Feminine singular: preguiçosa
Masculine plural: preguiçosos
Feminine plural: preguiçosas

Examples:

Ele está a ser preguiçoso. – He is being lazy.
Elas estão a ser preguiçosas. – They (fem.) are being lazy.

Why is it quando não estuda and not quando ela não estuda?

Portuguese often drops the subject pronoun when it is clear from context and from the verb ending. This is called a “null subject” or “pro‑drop” language.

The subject ela was already mentioned at the beginning: Ela acha…
The verb estuda is 3rd person singular, which matches ela.
So ela can be omitted in the second clause:
- Ela acha que está a ser preguiçosa quando não estuda à noite.
  (She thinks she is being lazy when she doesn’t study at night.)

You can say quando ela não estuda à noite, and it isn’t wrong, but it’s usually unnecessary and less natural in such a short, clear sentence. Portuguese speakers tend to omit repeated subject pronouns unless they need emphasis or clarification.

Why is não estuda in the simple present, not não está a estudar?

In Portuguese, the simple present is very commonly used for habits and general situations, more than in English.

quando não estuda à noite
= when she doesn’t study at night (whenever that happens, as a habit or recurring situation)

If you say:

quando não está a estudar à noite

it sounds more like when she is not studying at night (at that moment), more specific and situational.

Here, the sentence is about a recurring pattern (on nights when she doesn’t study, in general), so não estuda in the simple present is the most natural form.

What is the difference between à noite, de noite, and na noite?

All three exist, but they don’t mean exactly the same:

à noite
- Very common for at night / in the evening in general.
- à is a contraction of a + a (preposition + article) with a grave accent.
- Example: Ela costuma estudar à noite. – She usually studies at night.
de noite
- Also at night, a bit more colloquial or neutral; often interchangeable with à noite.
- Example: Gosto de ler de noite. – I like reading at night.
na noite
- More specific: in the night (a particular night, a specific night-time context).
- na = em + a.
- Example: Na noite de sábado, houve muito barulho. – On Saturday night, there was a lot of noise.

In your sentence, à noite is a very natural choice to mean at night in a general, habitual way.

Why is the negative placed as não estuda and not something like estuda não?

The basic rule for negation in Portuguese is:

não + verb

So:

não estuda – does not study
não come – does not eat
não fala – does not speak

In standard European Portuguese, estuda não is not a normal way to say doesn’t study. You might see …não at the end in some dialectal or very informal speech (especially in Brazil, like ele estuda, não as a kind of tag), but for simple negation in a sentence like this, it must be não estuda.

Can I drop que and say Ela acha está a ser preguiçosa?

No. In Portuguese, you normally need que to introduce a subordinate clause after verbs like achar, pensar, dizer, saber, etc.

Correct:
- Ela acha que está a ser preguiçosa.
Incorrect:
- Ela acha está a ser preguiçosa. ✗

This que is similar to the that in English She thinks *that she is being lazy. In English, *that can often be omitted; in Portuguese, que usually cannot be omitted in this type of sentence.

Is there any difference in meaning between Ela acha que está a ser preguiçosa and Ela acha que é preguiçosa?

Yes, the difference is important:

Ela acha que está a ser preguiçosa
She thinks she is being lazy (in this situation / at this time).
→ Temporary behaviour, linked here to when she doesn’t study at night.
Ela acha que é preguiçosa
She thinks she is lazy (as a person, in general).
→ More of a permanent trait, part of her character.

In your full sentence, with quando não estuda à noite, the idea is that in those moments (on nights when she doesn’t study), she feels she is being lazy, not that she considers herself a lazy person overall. Hence está a ser, not é.