Эта минута казалась вечностью.

Because минута is a feminine noun in Russian. The demonstrative this has to agree with the noun in gender, number, and case.

• masculine: этот
• feminine: эта
• neuter: это
• plural: эти

So:

• эта минута = this minute

Минута is the grammatical subject of the sentence — it is the thing that seemed like something.

In Russian, the subject is usually in the nominative case. So:

• эта минута = this minute (subject)
• казалась = seemed
• вечностью = an eternity

In the past tense, Russian verbs agree with the subject in gender and number.

The subject here is минута, which is feminine singular, so the verb is:

• masculine: казался
• feminine: казалась
• neuter: казалось
• plural: казались

So Эта минута казалась... uses the feminine form because минута is feminine.

The dictionary form is казаться, which means to seem or to appear.

In this sentence:

• казаться → infinitive
• казалась → past tense, feminine singular

So казалась literally means seemed.

Because after казаться in this kind of sentence, Russian often uses the instrumental case for the thing something seems to be.

So:

• nominative: вечность = eternity
• instrumental: вечностью = as an eternity / like an eternity

This pattern is very common:

• Он казался уставшим. = He seemed tired.
• Она казалась врачом. = She seemed to be a doctor.
• Эта минута казалась вечностью. = This minute seemed like an eternity.

Very often, yes, when it means to seem like / to appear to be something.

Common patterns include:

• казаться + instrumental noun
• казаться + instrumental adjective
• казаться + clause

Examples:

• Он казался героем. = He seemed like a hero.
• Она казалась счастливой. = She seemed happy.
• Казалось, что время остановилось. = It seemed that time had stopped.

So in your sentence, вечностью is instrumental because it is what the minute seemed to be.

Not natural English. A more natural translation is:

• This minute seemed like an eternity.
• That minute felt like an eternity. (depending on context)

Russian often omits a separate word for like in this structure because the instrumental case already carries that idea.

So вечностью helps express like an eternity.

Russian past tense is much simpler than English tense systems. Казалась just means seemed in the past.

Russian does not usually make the same continuous/non-continuous distinction here that English can make. So one past-tense form covers what English might express as:

• seemed
• was seeming (rare in English anyway)

In normal English, seemed is the natural translation.

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

The neutral order here is:

• Эта минута казалась вечностью.

But other orders are possible for emphasis, for example:

• Вечностью казалась эта минута.
• Казалась вечностью эта минута.

These all keep the same basic meaning, but the emphasis changes. The original sentence is the most straightforward and neutral.

Yes. Минутка is a diminutive form, meaning something like a little minute or a moment, and it can sound more emotional, affectionate, or conversational.

Compare:

• Эта минута казалась вечностью. = This minute seemed like an eternity.
• Эта минутка казалась вечностью. = This little moment seemed like an eternity.

The original минута is more neutral and natural in a general statement.

Yes, but the meaning changes.

• Эта минута казалась вечностью. = This minute seemed like an eternity.
• Эта минута была вечностью. = This minute was an eternity.

Using казалась gives a subjective impression: it felt extremely long.
Using была is stronger and more direct, often more poetic or dramatic.

So казалась is a very natural choice for expressing perception.

It expresses that a very short period of time felt unbearably long. This is a common way to describe:

• waiting
• fear
• suspense
• pain
• anxiety
• emotional intensity

So even though минута literally means a minute, the sentence suggests a psychological experience of time: one minute felt endless.