Learn Hardcore Turkish: Kaplumbağa parkta yavaşça yürüyor. - The tortoise is walking slowly in the park.

The Turkish ğ is not a hard “g.” It mainly lengthens the preceding vowel or creates a very light glide. So kaplumbağa is roughly pronounced like “kap-lum-baa,” with a long final “a.”

Why is there no word for “the”?

Turkish has no articles like “the.” Definiteness is understood from context. If you want to say “a turtle,” you can add bir: Bir kaplumbağa parkta yavaşça yürüyor.

So is the subject here definite (“the turtle”) or indefinite (“a turtle”)?

Without bir, sentence-initial Kaplumbağa is usually read as definite (“the turtle”) or generic. To clearly introduce a new, indefinite subject, Turkish tends to use bir and often places it later in the sentence: Parkta bir kaplumbağa yavaşça yürüyor.

What exactly does parkta mean, and why is it -ta and not -da?

-DA is the locative case (“in/at/on”). It has four forms: -da/-de/-ta/-te.

Choose a/e by vowel harmony: last vowel back (a, ı, o, u) → a; front (e, i, ö, ü) → e.
Choose d/t by voicing: after a voiceless consonant (f, s, t, k, ç, ş, h, p), use t; otherwise d. Since “park” ends with voiceless k and its last vowel is back (a), you get parkta (“in/at the park”).