Essa época foi difícil.

Because época is a feminine noun in Portuguese.

• essa = feminine singular
• esse = masculine singular

So:

• essa época
• but esse período

A good habit is to learn nouns together with their gender.

Essa is a demonstrative word, meaning something like that or this, depending on context.

In Brazilian Portuguese, essa is very commonly used in everyday speech for something already mentioned, known from context, or mentally present in the conversation.

So essa época often means:

• that time
• that period
• that phase

Even if English might use that, Portuguese very naturally uses essa here.

These are all demonstratives, but they point to things differently.

Traditional distinction:

• esta = this, near the speaker
• essa = that, near the listener or already mentioned
• aquela = that over there, farther away from both

In modern Brazilian Portuguese, though, essa often replaces esta in normal conversation.

So in real Brazilian usage:

• essa época is very natural
• esta época is possible, but sounds more formal or careful
• aquela época would usually mean that time back then, often more distant in memory or history

Época usually means period, era, time, or phase, depending on context.

It is often used for:

• a period in someone’s life
• a historical period
• a general stretch of time

Examples:

• Naquela época, eu morava no Rio. = At that time, I lived in Rio.
• Foi uma época complicada. = It was a complicated period.

It does not usually mean clock time. For that, Portuguese uses hora.

This is a very common and important question.

Both can translate as was, but they are used differently:

• foi = preterite, presenting the period as a completed whole
• era = imperfect, describing an ongoing background situation

So:

• Essa época foi difícil.
  = That period was difficult.
  This treats the whole period as a complete unit.

• Essa época era difícil.
  This is less natural in many contexts, because it sounds more like background description without clear boundaries.

In Portuguese, when talking about an entire period and evaluating it as a whole, foi is often the natural choice.

Foi can come from either ser or ir.

• ser → foi = was
• ir → foi = went

In this sentence, it must be from ser, because difícil is an adjective describing essa época.

So:

• Essa época foi difícil. = That period was difficult.

You could not interpret it as went difficult. The sentence structure makes ser the only logical reading.

Because essa already works as a determiner before the noun.

In Portuguese, demonstratives like esse/essa/este/esta/aquele/aquela normally come directly before the noun and do not need an article:

• essa época
• esse livro
• aquela casa

You may sometimes see combinations like nessa época or naquela época, but that is different:

• nessa época = in that period / at that time
• naquela época = in that time back then

Those include a preposition plus article fused into one word.

Because difícil is an adjective with the same form for masculine and feminine singular.

So you get:

• um período difícil
• uma época difícil

No change there.

It only changes in the plural:

• períodos difíceis
• épocas difíceis

Many Portuguese adjectives behave like this.

They mainly show stress.

• época → stress on the É
• difícil → stress on the fí

Approximate pronunciation:

• época ≈ EH-po-ka
• difícil ≈ jee-FEE-sew in Brazilian Portuguese, though the final l can sound like a soft w

The accents help you know where the strong syllable goes. They are part of the correct spelling and should not be omitted in standard writing.

They are similar, but not always identical.

• essa época = that period / that time
• esse tempo = that time, but tempo is broader and can also mean weather
• essa fase = that phase, often more personal or emotional
• esse período = that period, a bit more neutral or formal

Examples of nuance:

• Essa época foi difícil. = natural, broad, common
• Essa fase foi difícil. = more like a stage in life
• Esse período foi difícil. = slightly more formal or objective

So yes, they can overlap, but época has its own feel.

Yes, absolutely.

The difference is mostly in perspective:

• essa época often refers to a time already under discussion or relatively close in the conversation
• aquela época often feels more distant, like back then or that period back in those days

So both can work, but they are not always interchangeable in tone.

Compare:

• Essa época foi difícil. = That time was difficult.
• Aquela época foi difícil. = That time back then was difficult.

Yes, it is very natural.

Essa época foi difícil. sounds like something a native speaker could easily say when talking about:

• childhood
• a breakup
• financial problems
• a historical period
• a stressful phase of life

It is short, simple, and idiomatic. A Brazilian speaker might also say:

• Naquela época foi difícil.
• Foi uma época difícil.
• Essa fase foi difícil.

But your sentence is completely natural as it stands.