Learn Turkish: Sen neredesin?

In Turkish, the concept of 'to be' is attached to the end of words as a suffix. Nerede means 'where,' and -sin is the personal suffix for 'you are.' So, neredesin literally translates to 'where-are-you.'

Since -sin already means 'you are,' do I actually need to include sen?

No, you can drop it! Turkish often leaves out the pronoun because the suffix at the end of the sentence already tells us who the subject is. You can simply ask Neredesin? Adding Sen just gives a bit of extra emphasis, like asking 'Where are you?'

Is the -de in nerede the same suffix we use to say 'in' or 'at' a location?

Exactly! The base word here is nere (what place). By adding the location suffix -de (in/at/on), it becomes nerede, which literally means 'at what place.'

How did we know to use -sin instead of another form like -sun or -sın?

This is due to vowel harmony. The personal suffix for 'you' has four forms: -sın, -sin, -sun, and -sün. Because the last vowel in nerede is an 'e', the suffix takes the matching 'i' form, becoming -sin.