Learn Finnish: Ohjaaja kysyi, miksi en venytellyt. - The instructor asked why I didn't stretch.

The verb venytellä (to stretch) is a Type 3 verb. To make the form used with negative verbs like en (the active past participle), you find the consonant stem by dropping the final two letters: venytel-. Usually, you add -nut or -nyt. But because the stem ends in l, the n assimilates (changes to match) the l. So, venytel- + -nyt becomes venytellyt. The same thing happens with r and s stems in Type 3!

I know en means 'I don't', so how does en venytellyt become 'I didn't stretch'?

In Finnish, the negative word (en, et, ei, etc.) is exactly the same in both the present and past tense! To make it past tense, you change the main verb. Instead of the present tense form (en venyttele), you use the active past participle (venytellyt). The combination of en and the participle is what tells you it happened in the past.

How do we form the past tense kysyi from the verb kysyä?

Kysyä (to ask) is a Type 1 verb. To form the past tense, you take the stem kysy- and add the past tense marker -i. For Type 1 verbs ending in -u or -y, the vowel doesn't disappear or change when adding the past tense marker. You just stick the -i right on the end, giving you kysyi.