Learn Finnish: Tämä jälki jätettiin yöllä.

The basic verb is jättää (to leave, a Type 1 verb). To form the passive past, you can think of the passive present form (jätetään), remove the final -ään, and replace it with -iin. This gives you jätettiin.

Why is there a single t in the first part of jätettiin, but a double tt later on?

The verb jättää undergoes consonant gradation. The passive voice always uses the "weak" stem, so the original double tt weakens to a single t (jäte-). The double tt you see later on actually belongs to the passive past ending itself (-ttiin), which is attached to that weak stem.

"This trace" is the object being left. Why is Tämä jälki in the basic dictionary form instead of having an -n object ending?

This is a very important rule for the passive voice! When a sentence is in the passive, its whole object does not take the usual -n ending (the accusative/genitive shape). Instead, it stays in the nominative (dictionary) form. If the sentence were active ("I left this trace"), it would be Jätin tämän jäljen.

Why is the ending -llä used in yöllä (at night)?