Learn Finnish: Myrsky jatkui metsässä. - The storm continued in the forest.

The dictionary form is jatkua (to continue), which is a Type 1 verb. First, find the stem by dropping the final -a, giving us jatku-. Because the stem ends in a short -u, we simply add the past tense marker -i-. Since "the storm" is "it" (third-person singular), there is no personal ending to add at the end, leaving us with exactly jatkui.

Why didn't the -u in the stem disappear when we added the past tense -i?

When a Type 1 verb's stem ends in the short vowels -o, -ö, -u, or -y, that vowel stays and the past tense -i- is just added right next to it. You only replace or drop the final stem vowel if it is -a, -ä, or -e. So jatku- simply becomes jatkui.

Is jatkua the same verb I would use if I wanted to say "I continued the game"?

No, and this is a common mix-up! Jatkua is an intransitive verb, meaning it describes something continuing on its own (like the storm). If you want to say that you continued doing something, you must use the transitive verb jatkaa.

Why does metsässä end with -ssä instead of -ssa?