Секундомер показывает одну минуту.

Because одну минуту is the direct object of the verb показывает (shows).

• одна минута = nominative case, used for the subject
• одну минуту = accusative case, used for a direct object

In this sentence, the subject is секундомер, and the thing being shown is one minute, so one minute has to be in the accusative.

Because минута is a feminine noun.

The numeral one changes to match the noun in gender, number, and case:

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

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

So:

• nominative: одна минута
• accusative: одну минуту

Минуту is in the accusative singular.

That is because показывать is a transitive verb and normally takes a direct object in the accusative.

So the structure is:

• Секундомер = subject, nominative
• показывает = verb
• одну минуту = direct object, accusative

Показывает is:

• present tense
• 3rd person singular
• from the verb показывать

So it means:

• shows
• is showing

In this sentence, English would usually say The stopwatch shows one minute or The stopwatch reads one minute.

Because показывать is the imperfective verb, which is the normal choice for describing what something displays or indicates.

A stopwatch display is a state/current reading, not really a one-time completed action. So Russian naturally uses:

• показывает = shows / is showing

The perfective показать usually means to show once, to demonstrate, or to reveal, and would sound less natural here.

Yes. In this context, показывает means shows, displays, or reads.

With clocks, watches, thermometers, meters, and similar devices, Russian often uses показывать:

• Часы показывают пять часов. = The clock shows five o’clock.
• Термометр показывает тридцать градусов. = The thermometer shows thirty degrees.

So here Секундомер показывает одну минуту means the stopwatch display reads 1:00.

Yes. Russian has no articles.

So секундомер can mean either:

• the stopwatch
• a stopwatch

The exact meaning depends on context. In an isolated sentence, English often uses the stopwatch, but Russian itself does not mark that difference.

Yes. Russian word order is more flexible than English because the case endings already show each word’s role.

The neutral, straightforward order is:

• Секундомер показывает одну минуту.

But you could change the order for emphasis, for example:

• Одну минуту показывает секундомер.

That sounds more marked and emphasizes one minute. It is grammatical, but less neutral.

Yes, that is possible, but it changes the feel slightly.

• Секундомер показывает одну минуту = explicitly one minute
• Секундомер показывает минуту = shows a minute / shows minute one, depending on context, and can sound less precise as a standalone teaching example

When you want to state the exact reading clearly, одну минуту is the most natural choice.

Because many feminine nouns ending in -а change to -у in the accusative singular.

So:

• nominative: минута
• accusative: минуту

This is a very common pattern:

• книга → книгу
• машина → машину
• лампа → лампу

So минута → минуту follows a regular rule.

This is an important Russian pattern:

• одну минуту = one minute
• две минуты = two minutes
• три минуты = three minutes
• четыре минуты = four minutes
• пять минут = five minutes

So after 1, the noun is singular and the numeral agrees with it. After 2, 3, 4, Russian uses a different form. After 5 and higher, the noun usually appears in a plural form like минут.

This is one of the trickier number patterns in Russian, so it is very normal to wonder about it here.

The main word stress is:

• секундоме́р
• пока́зывает
• одну́
• мину́ту

So roughly:

секундоме́р пока́зывает одну́ мину́ту

This can help you avoid one of the most common learner problems in Russian: putting the stress on the wrong syllable.