Это была глупость.

In this pattern, это был / была / было ..., the past-tense form matches the noun that names what это refers to.

Here that noun is глупость, and глупость is feminine singular, so Russian uses была.

• Это была глупость. = That was a foolish thing / That was stupidity.
• Это было письмо. = That was a letter.
  (письмо is neuter, so было)

So была is chosen because of глупость, not because of это.

Because это here is a standalone word meaning something like this / that / it, not an adjective directly modifying глупость.

Compare:

• Это была глупость. = That was a foolish thing.
• Эта глупость... = This foolish thing / this nonsense...

In эта глупость, эта agrees with the feminine noun глупость.

But in Это была глупость, это does not agree with глупость in that way. It points to a situation, action, statement, or thing.

Russian leaves out to be only in the present tense.

So:

• Это глупость. = This/That is foolishness.
• Это была глупость. = This/That was foolishness.
• Это будет глупость. = This/That will be foolishness.

So in the past tense, you do need был / была / было / были.

It is in the nominative singular.

In sentences of identification like Это была глупость, Russian normally uses the nominative for the noun after это.

That is why you get:

• Это была ошибка.
• Это был мой брат.
• Это было чудо.

For many learners, this feels unusual because they may know that predicate nouns after быть are often instrumental in other patterns, such as:

• Он был врачом. = He was a doctor.

But with это-sentences like this one, nominative is the normal choice.

Глупость can mean a few related things, depending on context:

• stupidity as an abstract idea
• a foolish thing
• nonsense

In Это была глупость, it often means that was a foolish thing to do/say or simply that was stupid.

So the noun is broader than just one English word-for-word translation.

Yes, it can.

Глупость is sometimes used for something foolish, and sometimes for something silly or nonsensical that someone said.

So depending on context, Это была глупость could mean:

• That was stupid.
• That was a foolish thing.
• That was nonsense.

If someone is reacting to an idea or statement, nonsense may fit well. If they are reacting to an action, a foolish thing or that was stupid is more likely.

Yes, and it is very natural.

• Это была глупость.
• Это было глупо.

Both can mean That was stupid, but the nuance is a little different:

• Это было глупо = more direct, using a predicative word meaning stupid/foolish
• Это была глупость = uses a noun, so it can sound a bit more categorical, like That was a foolish act / That was sheer stupidity

Very often, both are possible. The noun version may feel a little stronger or more concrete.

Это can refer to almost anything that has just been mentioned or is obvious from the situation:

• something someone said
• something someone did
• an event
• a whole situation
• an idea

So это is very flexible. In English, it may become this, that, or it, depending on context.

For example:

• Someone tells you what happened. You reply: Это была глупость.
• You see a bad decision being made. You say: Это была глупость.

In both cases, это points to the whole thing.

The normal, neutral order is:

Это была глупость.

Russian word order is flexible, but changing it changes the emphasis.

For example:

• Это была глупость. = neutral
• Глупость это была. = marked, emphatic, more like It was stupidity, that’s what it was

As a learner, the basic order is the one you should use unless you have a specific reason to emphasize something.

You usually know from context, because Russian has no articles like a and the.

So глупость by itself does not tell you whether English should use:

• a
• the
• no article at all

In real English, the most natural translation of Это была глупость is often not the most literal one. Instead of It was foolishness, English often prefers:

• That was stupid.
• That was a foolish thing to do.
• That was nonsense.

So context and natural English style matter more than a word-for-word match.

The stress is:

Э́то была́ глу́пость.

So:

• э́-то
• бы-ла́
• глу́-пость

That stress pattern is worth learning, because Russian stress is unpredictable and must often be memorized word by word.