Learn Hardcore Portuguese (Portugal): Ontem eu estava de mau humor e a conversa com o Pedro foi chata. - Yesterday I was in a bad mood and the conversation with Pedro was boring.

You wouldn’t normally say em mau humor or mal humor. Those forms sound unnatural or influenced by English. Think of de bom/mau humor as a fixed idiomatic structure with de.

What is the difference between mau and mal?

They look similar but behave differently:

mau is an adjective (modifies nouns) and agrees in gender and number:
- mau humor (bad mood) – masculine singular
- má ideia (bad idea) – feminine singular
- maus resultados (bad results) – masculine plural
mal is usually an adverb (modifies verbs, adjectives, or other adverbs), roughly “badly” or “poorly”:
- Ele fala mal. – He speaks badly.
- Ela dormiu mal. – She slept badly.

In de mau humor, mau is describing humor (a noun), so you must use the adjective mau, not mal.

Why is it eu estava and not eu estive?

Both are past tenses of estar, but they express different types of past:

estava = imperfect (ongoing / background / repeated past)
estive = preterite (completed, one-off event)

In Ontem eu estava de mau humor, the speaker is describing a general state they were in during that period yesterday, not a single completed event. The imperfect fits better:

Ontem eu estava de mau humor. – Yesterday I was (generally) in a bad mood.

If you used estive here (Ontem eu estive de mau humor), it would sound odd or overly “punctual”, as if being in a bad mood were a short, single action with a clear start and end, which is not how people normally talk about moods.

Why is it foi chata and not era chata or estava chata?

Again, this is about aspect (how you view the past):

foi chata – preterite of ser; treats the conversation as a complete event and judges it as boring as a whole.
era chata – imperfect of ser; would give a more descriptive, ongoing background sense (like “it used to be boring” / “it was boring over a period”).
estava chata – imperfect of estar; would emphasise a temporary state of the conversation at that time.

In context:

…a conversa com o Pedro foi chata.
→ The conversation (as an event, from start to finish) was boring.

Using era chata here is possible but would usually sound more like describing what the conversation was like in general, not judging one specific past event. Foi chata is the most natural choice when you’re looking back at one specific conversation that’s finished.

Why is it a conversa com o Pedro and not a conversa com Pedro?

In European Portuguese, it is very common (and often more natural) to use a definite article before personal names:

o Pedro, a Ana, o João, a Marta, etc.

After a preposition, the article is still used:

com o Pedro – with Pedro
para a Ana – for Ana
da Marta – of/from Marta

In many varieties of Brazilian Portuguese, people often omit the article before names (saying com Pedro, com Ana), but in Portugal com o Pedro sounds more natural and is standard.

Why is chata feminine? Why not chato?

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

a conversa – conversa is a feminine noun (you can see that from the article a).
Therefore, the adjective must be feminine: chata.

Patterns with this adjective:

um filme chato – a boring film (masculine singular)
uma conversa chata – a boring conversation (feminine singular)
filmes chatos – boring films (masculine plural)
conversas chatas – boring conversations (feminine plural)

So: a conversa … foi chata, not foi chato.

Can I drop eu and just say Ontem estava de mau humor…?

Yes. Portuguese (including European Portuguese) is a “null-subject” language: subject pronouns are often omitted when the verb ending makes the subject clear.

(Eu) estava de mau humor.
(Eu) fui ao cinema.

In Ontem estava de mau humor, the form estava already tells you the subject is first person singular (I), so eu is optional. Including eu can add a bit of emphasis or clarity, but grammatically both are correct:

Ontem eu estava de mau humor… – slightly more explicit/emphatic.
Ontem estava de mau humor… – more neutral and very natural in everyday speech.

Does Ontem always have to go at the beginning? Could I say Eu ontem estava de mau humor or Eu estava de mau humor ontem?

You can move ontem; word order is flexible for time expressions. All of these are possible:

Ontem eu estava de mau humor…
Eu ontem estava de mau humor…
Eu estava de mau humor ontem…

Differences:

Ontem eu estava… – puts more emphasis on “yesterday” as the time frame.
Eu estava de mau humor ontem… – sounds a bit more neutral, like everyday speech.
Eu ontem estava… – also correct, but feels slightly more marked in style (you might hear it in speech, but less often than the other two).

So ontem doesn’t have to be at the beginning, but it often appears there.

Should there be a comma after Ontem? Why is there none in the sentence?

Both are seen in practice:

Ontem, eu estava de mau humor…
Ontem eu estava de mau humor…

A comma after a short, single-word time adverb like Ontem is optional. In modern informal writing (and normal speech punctuation), it is very common to leave it out.

You’d be more likely to use a comma when the initial phrase is longer or more complex:

Ontem à noite, eu estava de mau humor…
Na semana passada, tive muito trabalho.

Why is it a conversa and not a conversação? Is there a difference?

Yes, there is a difference in usage:

a conversa is the everyday, natural word for “conversation”, “chat”:
- Tive uma conversa com o Pedro. – I had a conversation with Pedro.
a conversação exists, but it’s more formal, technical, or limited to certain fixed expressions (e.g. aula de conversação = conversation class). It’s not what you normally use for “a chat”.

So in this sentence, a conversa com o Pedro is exactly what a native speaker would say.

What nuances does chata have? Does it just mean “boring”?

In this context, chata indeed means “boring” or “tedious”, but it can carry a range of similar ideas:

boring
dull
tedious
annoying / a bit of a drag

For a conversa foi chata, a natural translation could be:

“the conversation with Pedro was boring”
“the conversation with Pedro was a drag”

It usually has a mildly negative, somewhat informal tone. It doesn’t sound very formal, but it’s very common in everyday European Portuguese.