Градусник показал высокую температуру, и я остался дома.

Because градусник is the subject of the sentence — it is the thing that showed something.

• Градусник показал... = The thermometer showed...

In Russian, the subject of a verb is usually in the nominative case.

Because высокую температуру is the direct object of показал.

The verb показать / показывать often takes an object in the accusative case:

• показал что? → высокую температуру

So both words change to match the feminine singular accusative:

• высокая → высокую
• температура → температуру

This is why you do not see nominative высокая температура here.

Because adjectives must agree with the nouns they describe in gender, number, and case.

Here:

• температура is feminine singular
• it is in the accusative case
• so the adjective must also be feminine singular accusative

That gives:

• nominative: высокая температура
• accusative: высокую температуру

This is a very common pattern with feminine adjectives.

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

The subject is градусник, which is a masculine singular noun, so the verb is:

• показал = masculine singular past

Compare:

• градусник показал = the thermometer showed
• машина показала = the machine showed
• часы показали = the clock/watch showed

So the ending tells you something about the subject.

The verb here is остаться — a verb meaning to remain / to stay.

Its past masculine form is:

• остался

This form includes the reflexive particle -ся, which is part of the verb itself.

So:

• infinitive: остаться
• past masculine: остался
• past feminine: осталась
• past plural: остались

You cannot remove -ся, because остаться and остать are not the same thing. In fact, остать is not the normal verb here.

Because in Russian past tense, the verb agrees with the speaker’s gender when the subject is я.

So:

• a male speaker says я остался дома
• a female speaker says я осталась дома

English does not show this difference, but Russian does.

И means and. It joins the two parts of the sentence:

• Градусник показал высокую температуру
• я остался дома

So the full structure is:

• The thermometer showed a high temperature, and I stayed home.

It is simply connecting two actions/events.

Because и is joining two independent clauses, each with its own subject and verb:

1. Градусник показал высокую температуру
2. я остался дома

In Russian, when two full clauses are connected by и, a comma is usually used.

If и were joining just words, not full clauses, there would normally be no comma:

• чай и кофе = tea and coffee

But here it joins two complete thoughts, so the comma is correct.

Дома is a very common adverb meaning at home.

So:

• я остался дома = I stayed home / I stayed at home

This is usually more natural than в доме, which means more literally in the house/building.

Compare:

• Я дома. = I am at home.
• Я остался дома. = I stayed home.
• Я в доме. = I am inside the house.

So дома refers to being at one’s home, while в доме emphasizes physical location inside a building.

Because дома is functioning as an adverb, not as a noun with a preposition.

Russian often uses certain location words this way:

• дома = at home
• здесь = here
• там = there

So you simply say:

• остаться дома
• быть дома

No preposition is needed.

Показал is the perfective past form, and it presents the action as a completed result:

• the thermometer gave a reading
• that reading was high

This fits the context well, because the result led to the next action:

• ..., и я остался дома

Показывал is imperfective and would suggest repeated action, ongoing action, or background description:

• Градусник показывал высокую температуру could mean The thermometer was showing a high temperature or used to show one.

That is possible in some contexts, but показал is the more natural choice when you mean a single completed reading.

Yes, in this context температура usually means temperature, and very often specifically body temperature / fever when talking about health.

So высокая температура can mean:

• high temperature
• a fever

Since the sentence also mentions a thermometer and staying home, the health meaning is very clear.

Yes. That is also very natural, but it means something slightly different in structure.

• Градусник показал высокую температуру... focuses on the thermometer’s reading.
• У меня была высокая температура... focuses directly on your condition: I had a high temperature / fever.

Both are correct. The original sentence is just framed through the result shown by the thermometer.

No, Russian word order is more flexible than English, though the original order is very natural.

The sentence:

• Градусник показал высокую температуру, и я остался дома.

is neutral and clear.

You could change word order for emphasis, for example:

• Высокую температуру показал градусник, и я остался дома.

But that sounds more marked, as if emphasizing high temperature specifically.

For most learners, the original word order is the best version to use.

Yes, градусник is the common everyday word for a thermometer, especially a medical thermometer.

There is also the more technical/international word:

• термометр

But in ordinary speech, especially about checking for fever, градусник is extremely common and natural.