Learn Hardcore Turkish: Seher olunca kuş sesleri bahçeyi renklendiriyor. - At dawn, the sounds of birds color the garden.

Seher means “dawn.”
The suffix -unca attaches to a verb (here ol-, “to become/arrive”) to form a temporal conjunction meaning “when.”
So seher olunca literally means “when dawn comes” or “once it becomes dawn.”

Why not use olduğunda or olursa instead of olunca?

olunca and olduğunda both mean “when,” but -unca is a bound conjunctional suffix (faster, more idiomatic) while olduğunda is a full participle + case ending (“when it has become”).
olursa means “if it becomes,” so that’s conditional, not purely temporal.
In everyday Turkish, olunca is the most natural for “when X happens.”

Can you break down renklendiriyor into its parts?

Certainly:

renk – “color” (noun root)
-li – adjective suffix “having/with” → renkli “colorful”
-dir – causative suffix “to make/cause to be” → renklendir- “to make colorful”
-iyor – present continuous suffix “is/are …ing” → renklendiriyor “is making (something) colorful,” i.e. “is coloring”

Why is bahçeyi in the accusative case with -yi?

Renklendirmek (“to color, to make colorful”) is a transitive verb that takes a direct object.
Turkish marks definite direct objects with the accusative suffix -i/-ı/-u/-ü, here producing bahçe + yi = bahçeyi (“the garden”).
Omitting -yi would make it indefinite (“a garden”) or unmarked, but because we mean “the garden,” we use bahçeyi.

Why doesn’t kuş sesleri have any case ending?

In Turkish, subjects (nominative) are unmarked.
kuş sesleri (“bird sounds”) is the subject of renklendiriyor, so it stays in the bare form with only the plural suffix -leri.

Why is the verb renklendiriyor placed at the end of the sentence?

Turkish is an SOV (Subject-Object-Verb) language:

Subordinate clause first (Seher olunca),
Then subject (kuş sesleri),
Then object (bahçeyi),
And finally the verb (renklendiriyor).
Putting the verb last is the normal word order in Turkish main clauses.

Why not use bahçede (“in the garden”) instead of bahçeyi?

bahçede would be locative (“in the garden”) and would describe where something happens.
Here the garden is what gets “colored,” so it’s the direct object, not a location.
That direct-object role demands the accusative bahçeyi.

Why is seher used here instead of sabah or şafak?

şafak strictly means “the first light of dawn,” often poetic.
seher is also literary/poetic for “dawn.”
sabah is the everyday word for “morning.”
All three refer to early day, but seher gives a more atmospheric, slightly elevated tone.