Термометр вимірює температуру.

Because температуру is the accusative singular form, used for the direct object of the verb.

• температура = nominative singular, the dictionary form
• температуру = accusative singular

In this sentence, temperature is the thing being measured, so it must be in the accusative:

• Термометр = the subject
• вимірює = measures
• температуру = the object

This is very common with feminine nouns ending in -а:
-а usually changes to -у in the accusative singular.

вимірює is the 3rd person singular present tense form of вимірювати (to measure).

It agrees with термометр, which is singular:

• я вимірюю = I measure
• ти вимірюєш = you measure
• він / вона / воно вимірює = he / she / it measures

So Термометр вимірює... literally means The thermometer measures...

Ukrainian does not have articles like English a/an and the.

So Термометр вимірює температуру could mean:

• A thermometer measures temperature
• The thermometer measures the temperature

The exact meaning depends on context.
Without context, this sentence sounds like a general statement: A thermometer measures temperature.

No. Ukrainian word order is fairly flexible because case endings show the grammatical roles.

The most neutral order here is:

• Термометр вимірює температуру.

But you could also hear:

• Температуру вимірює термометр.

That version puts more emphasis on thermometer, as if contrasting it with something else.

So the basic meaning stays the same, but word order can change the focus or emphasis.

Because Ukrainian verb endings already show the person and number, and when the subject noun is present, a pronoun is unnecessary.

Here, термометр is already the subject, so adding він would usually be redundant:

• Термометр вимірює температуру. = normal
• Він вимірює температуру. = It measures temperature
• Термометр, він вимірює температуру. = possible only in special emphatic contexts

In normal speech, Ukrainian usually avoids extra subject pronouns unless they are needed for emphasis or contrast.

Термометр is masculine.

A useful rule is that many Ukrainian nouns ending in a consonant are masculine.
Since термометр ends in -р, it is masculine.

That matters because verbs in the present tense agree with the subject in number and person, and in some other structures gender also becomes important.

For this sentence, the key point is simply that термометр is a singular noun meaning thermometer.

вимірює comes from вимірювати, which is imperfective.

The imperfective is used here because the sentence expresses a general fact or ongoing function:

• Термометр вимірює температуру. = A thermometer measures temperature.

If you used the perfective verb, it would sound more like a single completed measurement or a future completed action, not a general truth.

So for definitions, habits, and regular functions, the imperfective is the normal choice.

Yes. Ukrainian present tense does not make the same strict distinction English makes between:

• measures
• is measuring

So Термометр вимірює температуру can mean:

• A thermometer measures temperature
  or
• The thermometer is measuring the temperature

Context tells you which one is meant.
Without context, it is most naturally understood as a general statement.

Sometimes yes, but вимірює is the more precise and standard choice here.

• вимірювати / вимірює = to measure, in a clear and formal/neutral sense
• міряти / міряє = can also mean to measure, but it can be more colloquial and in some contexts can mean to try on clothes

So for a textbook-style sentence about what a thermometer does, вимірює is the best choice.

Yes, they are very close cognates:

• термометр = thermometer
• температура = temperature

That makes the sentence easier to recognize for English speakers.
The main new part to pay attention to is the grammar, especially:

• the verb form вимірює
• the accusative object температуру

So the vocabulary may feel familiar, but the endings are doing important grammatical work.