isya ga kensa wo setumeisimasita.

They mark the roles of the nouns:

• 医者が = the doctor is the subject, or the person doing the action
• 検査を = the examination / test is the thing being explained

So the structure is:

• 医者 = doctor
• が = subject marker
• 検査 = examination, test, inspection
• を = object marker
• 説明しました = explained

Both are possible, but they feel slightly different.

• 医者が検査を説明しました。 puts focus on the doctor as the one who explained it.
• 医者は検査を説明しました。 would sound more like as for the doctor, he/she explained the examination, perhaps contrasting with someone else.

So が is often used when identifying who did something, while は is more topic-based.

説明しました comes from:

• 説明 = explanation
• する = to do

Together, 説明する means to explain.

So:

• 説明します = explain / will explain
• 説明しました = explained

This is a very common Japanese pattern: a noun plus する makes a verb.

It is past polite.

• 説明します = explain / will explain
• 説明しました = explained

So the sentence is talking about something that already happened.

Yes. In Japanese, 説明する can take the thing being explained with を.

So:

• 検査を説明しました = explained the examination / test

You may also see sentences where the person receiving the explanation is marked with に:

• 患者に検査を説明しました。 = explained the examination to the patient

In your sentence, the listener is simply omitted.

As written, 検査 most naturally means the examination/test itself.

So the sentence means something like:

• The doctor explained the test.
• The doctor explained the examination.

If you wanted to say test results, Japanese would usually say something more specific, such as 検査結果.

Not completely. Japanese word order is more flexible than English because particles show each word’s role.

This sentence could also appear as:

• 検査を医者が説明しました。

That still means roughly the same thing, though the emphasis may change.
The most neutral basic order is often:

subject + object + verb

So your sentence is a standard, natural order.

The sentence does not say.

Japanese often leaves out information if it is clear from context. So this sentence only tells us:

• who did the explaining: the doctor
• what was explained: the examination/test

If you want to include the person who received the explanation, you can add に:

• 医者が患者に検査を説明しました。
  The doctor explained the examination to the patient.

They are read:

• 医者 = いしゃ
• が = が
• 検査 = けんさ
• を = を (pronounced o)
• 説明しました = せつめいしました

A full reading is:

いしゃ が けんさ を せつめいしました。

Japanese does not usually use articles like a or the.

So 医者 can mean:

• a doctor
• the doctor

And 検査 can mean:

• a test
• the test
• an examination
• the examination

Which one sounds best depends on context and the translation you are given.

No. Standard Japanese is usually written without spaces.

So this sentence would normally be written as:

医者が検査を説明しました。

Spaces are often added only in beginner materials to make the sentence easier to read.