A piada foi engraçada.

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

• piada is a feminine singular noun.
• So a piada = the joke

In Portuguese, articles are used very often, more often than in English.

Examples:

• A piada = the joke
• Uma piada = a joke

In Portuguese, every noun has a grammatical gender, usually masculine or feminine. Piada is simply a feminine noun, so words that go with it often have to match.

That is why you get:

• a piada
• engraçada

Both are feminine singular to agree with piada.

Because adjectives usually agree with the noun in gender and number.

Since piada is:

• feminine
• singular

the adjective also becomes:

• feminine
• singular

So:

• masculine singular: engraçado
• feminine singular: engraçada
• masculine plural: engraçados
• feminine plural: engraçadas

Examples:

• O filme foi engraçado.
• A piada foi engraçada.
• As piadas foram engraçadas.

Foi can come from either ser (to be) or ir (to go), depending on context.

In this sentence, foi is from ser:

• A piada foi engraçada. = The joke was funny.

But in another sentence, it could be from ir:

• Ela foi ao mercado. = She went to the market.

So learners often have to use context to know which verb it is.

Foi is the preterite form of ser, and era is the imperfect form. Both can mean was, but they are used differently.

• foi = a completed event or evaluation
• era = background, ongoing description, habit, or repeated past situation

So:

• A piada foi engraçada.
  = The joke was funny.
  This sounds like a completed judgment about a specific joke or moment.

• A piada era engraçada.
  = The joke was funny / used to be funny / was funny in context
  This is less neutral here and depends more on a larger story or background.

For a simple statement about one joke, foi engraçada is the most natural choice.

Usually, foi engraçada is the most natural version.

Why?

• ser is commonly used to describe something as inherently or characteristically funny in that situation.
• estar engraçado/a can sometimes suggest a temporary state, a particular mood, or even something a bit unusual.

For example:

• Você está engraçado hoje. = You’re funny today. / You’re acting funny today.

With piada, foi engraçada is the normal way to say the joke was funny.

Because engraçada here is not directly modifying the noun inside the noun phrase. It is part of the predicate with the verb ser.

Structure:

• A piada = subject
• foi = verb
• engraçada = predicate adjective

So this works like:

• The joke was funny

You can also put an adjective after the noun in other structures:

• a piada engraçada = the funny joke

But that is a different structure:

• A piada foi engraçada. = The joke was funny.
• A piada engraçada acabou. = The funny joke ended.

A simple approximate pronunciation is:

• A → ah
• piada → pee-AH-duh
• foi → roughly foy
• engraçada → roughly engrah-SAH-duh

A fuller approximation for the whole sentence:

• ah pee-AH-duh foy engrah-SAH-duh

A couple of notes:

• In Brazilian Portuguese, the d in piada and engraçada often sounds a bit like j in job before an a? Actually not here. Before a, it stays a normal d sound.
• The stressed syllables are:
  • piAda
  • engraÇAda

Yes, but it changes the meaning slightly.

• A piada foi engraçada. = The joke was funny.
  This refers to a specific joke already known in context.

• Uma piada foi engraçada. = A joke was funny.
  This is grammatically possible, but it is less common unless you are contrasting it with other jokes or talking about one joke among many.

In normal conversation, if both speakers know which joke you mean, a piada is more natural.

Yes, it is grammatical and natural.

It sounds like a straightforward comment about a specific joke:

• someone told a joke
• you are evaluating it afterward
• A piada foi engraçada.

In everyday conversation, Brazilians might also say:

• A piada foi boa. = The joke was good.
• A piada foi muito engraçada. = The joke was very funny.

So your sentence is completely fine and natural.

The plural is:

• As piadas foram engraçadas.

Everything changes to match the plural noun:

• a → as
• foi → foram
• engraçada → engraçadas

So you can see agreement happening in several places.

Singular:

• A piada foi engraçada.

Plural:

• As piadas foram engraçadas.

Not always. Engraçado/a often means funny, but depending on context it can also mean:

• amusing
• entertaining
• odd
• strange in a mildly surprising way

Examples:

• A piada foi engraçada. = The joke was funny.
• Que engraçado! = How funny! / How strange!

So context matters. In your sentence, the meaning is clearly funny.