A derrota foi difícil.

Because a is the feminine singular definite article, meaning the.

In Portuguese, it is very common to use an article before a noun, especially when the noun is the subject of the sentence. So a derrota means the defeat or the loss.

If you said just derrota, it would usually sound incomplete in a normal sentence like this.

Yes. Derrota is a feminine noun, so it takes feminine articles:

• a derrota = the defeat
• uma derrota = a defeat

That is why the sentence uses a, not o.

Derrota usually means defeat or loss, especially in contexts like:

• sports
• war
• elections
• competitions
• personal setbacks

So it is stronger and more specific than a very general English word like loss in some contexts.

For example:

• a derrota do time = the team’s defeat
• a derrota na eleição = the defeat in the election

Foi is the preterite form of ser (and also of ir, depending on context).

Here, foi means was.

So:

• A derrota foi difícil. = The defeat was difficult.

The preterite is used because the sentence presents the defeat as a completed event and gives an overall judgment about it.

This is a very common question.

• foi gives the idea of a completed event, seen as a whole
• era gives more of a background, ongoing, descriptive feeling

So:

• A derrota foi difícil. = The defeat was difficult.
  → a complete event, viewed as a whole

• A derrota era difícil.
  → less natural in most situations; it sounds more like background description and usually needs more context

In this sentence, foi is the natural choice.

Because difícil has the same form for masculine and feminine in the singular.

Compare:

• o jogo foi difícil = the game was difficult
• a derrota foi difícil = the defeat was difficult

But in the plural, it does change:

• os jogos foram difíceis
• as derrotas foram difíceis

So difícil does agree in number, but not in gender in the singular.

Not always. Depending on context, difícil can also mean:

• hard
• tough
• painful
• challenging

So A derrota foi difícil could mean not only that the defeat was technically difficult, but also that it was emotionally tough.

Yes. In natural English, hard or tough may sound better depending on context.

Portuguese difícil often covers meanings that English expresses with different words:

• difficult
• hard
• tough

So if the meaning is emotional, The loss was hard or The defeat was tough may sound more natural in English.

A common Brazilian pronunciation is approximately:

ah deh-HO-tah foy jee-FEE-seew

A few notes:

• derrota has stress on ro
• foi sounds roughly like foy
• in many parts of Brazil, the di in difícil sounds like jee
• the final l in difícil often sounds like a w-like sound in Brazilian Portuguese

So difícil may sound close to jee-FEE-seew.

Yes, but the meaning changes.

• A derrota foi difícil = the defeat was difficult
  → a specific defeat, probably already known in context

• Uma derrota foi difícil = a defeat was difficult
  → less specific; it introduces one defeat among others

So a and uma are not interchangeable.

Yes, but the normal, neutral order is:

A derrota foi difícil.

That is the most natural order for a simple statement.

A different order can be used for emphasis, for example:

• Difícil foi a derrota.

This is more literary or emphatic, and not the usual basic way to say it.

Because estar and ser do not mean exactly the same thing.

• foi difícil = it was difficult, as an overall fact or evaluation
• estava difícil = it was being difficult / it was difficult at that moment

With a derrota, foi difícil is much more natural if you are judging the event as a whole.

A derrota estava difícil sounds odd in most situations, because a defeat is usually treated as a completed result, not a temporary state.