Он сказал ещё одну глупость.

Because глупость is a feminine singular noun, and один has to agree with it.

The forms of one are:

• masculine: один
• feminine: одна
• neuter: одно

But here the phrase is in the accusative case, so feminine одна becomes одну.

So:

• nominative: одна глупость
• accusative: одну глупость

Because глупость is an inanimate feminine noun of the third declension, and in the singular its nominative and accusative forms are the same.

So:

• nominative: глупость
• accusative: глупость

That is why only одна → одну changes, while глупость stays the same.

Here ещё means another, one more, or yet another.

So ещё одну глупость means:

• another foolish thing
• one more stupid remark
• yet another piece of nonsense

A useful thing to remember is that ещё is a very flexible word. In other contexts it can also mean:

• still
• yet
• more

But in this sentence, with одну, it clearly has the sense of one more / another.

Сказал is the perfective past of сказать, so it presents the action as completed: he said it, one finished utterance.

That fits well here, because the sentence refers to one specific remark.

Compare:

• Он сказал ещё одну глупость. = He said another stupid thing.
• Он говорил глупости. = He was saying stupid things / He used to say stupid things.

So сказал focuses on a single completed act of speaking, while говорил would sound more ongoing, repeated, or less bounded.

Yes. Сказать глупость is a very normal and common expression.

Literally it is to say a stupidity, but in natural English it usually becomes:

• to say something stupid
• to say something foolish
• to make a stupid remark

Russian often uses a noun like глупость where English prefers a phrase like something stupid.

Because Russian has no articles like a or the.

So Russian simply says:

• Он сказал глупость.

and the meaning is understood from context as something like:

• He said a stupid thing
• He said something stupid

In this sentence, одну does not work like an English article. It really means one / another one as part of ещё одну.

Yes. Russian word order is more flexible than English word order because case endings show grammatical roles.

The sentence Он сказал ещё одну глупость is a neutral, natural order.

But you could also say things like:

• Ещё одну глупость он сказал.
• Он ещё одну глупость сказал.

These versions are still understandable, but the emphasis changes:

• putting ещё одну глупость earlier highlights what he said
• keeping Он сказал ещё одну глупость sounds more neutral

So the original sentence is a very standard choice.

Russian often can drop subject pronouns when the subject is clear from context.

So in the right context, you could say:

• Сказал ещё одну глупость.

and it may still be understood as He said another stupid thing.

However, он is included when the speaker wants to:

• make the subject explicit
• contrast him with someone else
• avoid ambiguity
• simply use a full, neutral sentence

So Он сказал ещё одну глупость is completely normal.

Ещё is pronounced roughly like ish-CHO.

The ё is important because it shows the sound yo / o after palatalization and it also shows the stress here:

• ещё

In many printed texts, Russians often write е instead of ё, so you may see еще. But it is still usually understood and pronounced as ещё.

For learners, it is very helpful to remember the spelling with ё whenever possible.

Not necessarily. The English word stupidity can sound quite heavy or abstract, but Russian глупость is often used in ordinary everyday speech.

Depending on context, it can mean:

• a foolish thing
• a stupid remark
• nonsense
• something silly

So in this sentence, a natural English translation is often softer and more idiomatic than the literal He said another stupidity. English would more naturally say:

• He said another stupid thing.
• He made another foolish remark.