Learn Finnish: Poika on teltassa. - The boy is in the tent.

This is consonant gradation! When you add a closed ending like the inessive -ssa (meaning "in"), a double consonant in the word's stem often simplifies to a single consonant. So, the double tt in teltta drops to a single t, giving us teltassa.

Why is the ending -ssa instead of -ssä?

This is due to vowel harmony. The word teltta contains the vowels e and a. Because a is a back vowel, the ending must also use the back vowel form. Therefore, we attach -ssa rather than -ssä.

Is there a separate word for "the" in this sentence?

No, Finnish doesn't have articles like "a" or "the". The word poika can mean either "a boy" or "the boy" depending on context. Because it is the subject at the beginning of the sentence, it naturally translates to "The boy" here.

I know the word poika can undergo consonant gradation too. Why didn't its k change here?