Learn Finnish: Opiskelija keskusteli tentistä. - The student discussed the exam.

Keskustella is a Type 3 verb. To make the past tense (imperfect), drop the final -la to find the consonant stem (keskustel-), then add the past tense marker -i-. Because this is the third-person singular ("he/she/it") form, we do not add a personal ending at the end, leaving us with keskusteli.

Why is it tentistä and not just tentti or tenttiä?

The verb keskustella (to discuss / to converse) always requires the topic you are talking about to be in the elative case (-sta/-stä). In Finnish, you always discuss "about" or "out of" something, even though English just says "discussed the exam."

The dictionary word for exam is tentti. Why did it lose a t here?

This is consonant gradation in action! The strong consonant cluster -ntt- weakens to -nt- when an ending closes the syllable. Adding -stä creates a closed syllable, so tentti becomes the weak stem tenti- before the ending is attached.

Why does the ending use -stä instead of -sta?