Learn Turkish: Doktor düşünüyor. - The doctor is thinking.

The verb root for 'to think' is düşün-, which ends in a consonant. The present continuous suffix -yor needs a vowel to attach to. Following the 4-way vowel harmony rule, since the last vowel in düşün- is ü, the connecting vowel must also be ü, giving us düşünüyor.

Why isn't there a separate word for 'is' in this sentence?

In Turkish, the concept of 'is doing' is built right into the verb. The -yor suffix handles the English 'is ...-ing' part all by itself, so a separate word for 'is' is not needed.

How do we know who is thinking just by looking at the verb?

The person doing the action is shown by a personal suffix at the very end of the verb. Because düşünüyor has no extra ending after -yor, it is in the default third-person form, which means 'he', 'she', or 'it' is doing the action. Here, it correctly matches the subject doktor.

Where is the word for 'the' in this sentence?

Turkish does not have a direct word for 'the'. A noun standing alone, like doktor, usually translates to 'the doctor' based on context. If you wanted to say 'a doctor is thinking', you would use the word bir (a/an) and say Bir doktor düşünüyor.