Doktor bu teşhisi doğruladı.

The neutral Turkish word order here is Subject + Object + Verb:

• Doktor = subject
• bu teşhisi = object
• doğruladı = verb

So Turkish usually puts the verb at the end, unlike English.

Because it is the direct object of the verb, and it is definite/specific.

In Turkish, a definite direct object usually takes the accusative suffix.
Here:

• teşhis = diagnosis
• teşhisi = the diagnosis / this diagnosis, as a direct object

So the ending -i is the accusative marker.

Because bu makes the noun specific: this diagnosis.

In Turkish, specific direct objects normally take the accusative. So:

• bir teşhis = a diagnosis
• bu teşhisi = this diagnosis

Compare:

• Doktor bir teşhis doğruladı = The doctor confirmed a diagnosis.
• Doktor bu teşhisi doğruladı = The doctor confirmed this diagnosis.

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

So doktor can mean:

• the doctor
• a doctor
• sometimes just doctor, depending on context

The exact English translation depends on the situation, not on a separate Turkish article.

Doğruladı is in the simple past tense.

It is built like this:

• doğrula- = confirm / verify
• -dı = past tense ending

So doğruladı means confirmed.

Why -dı and not -di?
Because Turkish suffix vowels follow vowel harmony. The last vowel in doğrula- is a, so the past ending appears here as -dı.

The main clue is the ending on teşhisi.

• teşhisi has the accusative ending, so it is the object
• doktor has no such object marking, so it is the subject

In Turkish, case endings often tell you the role of a noun more clearly than word order alone.

Yes. Turkish word order is flexible, but changes usually add emphasis or change the focus.

For example:

• Doktor bu teşhisi doğruladı. = neutral
• Bu teşhisi doktor doğruladı. = emphasizes the doctor
• Doktor doğruladı bu teşhisi. = more marked, less neutral in standard style

The most neutral version is the original one, with the verb at the end.

Because teşhisi here is just teşhis + accusative.

Teşhisini usually includes a possessive idea, such as his/her/its diagnosis, plus the accusative.

So:

• teşhisi = the diagnosis / this diagnosis
• teşhisini = his/her/its diagnosis (as object)

Since the sentence means this diagnosis, not his/her diagnosis, teşhisi is the correct form.

The dictionary form is doğrulamak = to confirm / to verify.

To make the past tense:

1. remove -mak
2. keep the verb stem doğrula-
3. add the past ending -dı

So:

• doğrulamak → doğrula- → doğruladı

This is a very common pattern in Turkish verbs.

A few letters may stand out:

• ş = sh
• ğ = usually a softening/lengthening letter, not a strong g
• ı = a vowel with no exact English equivalent

Roughly:

• Doktor ≈ dok-tor
• teşhisi ≈ teh-shi-see
• doğruladı ≈ doh-ru-la-duh, but the last vowel is Turkish ı, not English ee

A helpful note:

• In doğru, the ğ usually does not sound like a hard g. It mostly smooths or lengthens the vowel before it.

Yes. Turkish often omits the subject if it is already clear from context.

So you could also say:

• Bu teşhisi doğruladı.

Depending on context, that could mean:

• He/she confirmed this diagnosis

In your sentence, doktor is included to make the subject explicit.