Learn Finnish: Kuulin kandidaatin kirjoittaneen gradun. - I heard that the bachelor had written the master's thesis.

In the referative construction (partisiippirakenne), the subject of the embedded clause ("the bachelor") is always put into the genitive case. This structure allows you to shorten a sentence by replacing a normal "that" clause.

How do we form kirjoittaneen and what time does it refer to?

It is the past active participle of kirjoittaa (to write). You form it by taking the normal -nut/-nyt participle (kirjoittanut) and changing the ending to -neen. Because it is the past participle, it shows that the bachelor wrote the thesis before the main action of hearing about it, which is why it translates to "had written".

How would this sentence look if I used a normal että (that) clause instead?

You would say: Kuulin, että kandidaatti oli kirjoittanut gradun. Using että is much more common in everyday speech. The -neen structure you see in this example is mostly found in formal writing, literature, and news reporting.

Why does the word gradun also end in -n?

Gradun is the direct object of the action. Even though the verb has been turned into a participle (kirjoittaneen), it still behaves like a normal verb and takes an object. Because the writing of the thesis is a completed action, the object takes the standard genitive-accusative -n ending.