Learn Turkish: Sen parktasın. - You are in the park.

This is because of consonant assimilation. The word park ends in the voiceless consonant k. When you add a suffix starting with d to a word ending in a voiceless consonant (like p, ç, t, k, f, h, s, ş), the first letter of the suffix hardens to match it. So, the d in the 'in/at/on' suffix -da changes to t, making it -ta.

Why use -ta instead of -te?

After changing the d to a t, we still have to follow the rules of 2-way vowel harmony. Since the only vowel in park is a, which is a back vowel, the suffix takes an a as well. If the word had a front vowel, it would be -te, like in markette (at the market).

What does the -sın at the end of parktasın mean?

The suffix -sın means 'you are'. In Turkish, the verb 'to be' is usually attached to the end of a word as a personal suffix. Because parkta ends with an a, 4-way vowel harmony tells us to use -sın instead of -sin, -sun, or -sün.

Could I just say Parktasın without the word Sen?