Learn Swedish: Bergakungen gömde guldet i berget. - The mountain king hid the gold in the mountain.

This is a compound word made of berg (mountain) and kungen (the king). The -a- is a linking vowel (fogevokal). While berg usually takes an -s- in modern compounds (like bergsbestigare, mountain climber), older or mythological words often use -a-. This -a- actually survives from an old plural possessive form, so it historically means "king of the mountains".

We see berg used as a compound in Bergakungen, but then as berget at the end. Why?

When a noun is the first part of a compound (like berg- in Bergakungen), it cannot be in the definite form. It drops the definite ending (-et or -en) and uses its stem, sometimes adding a linking vowel like we see here. At the end of the sentence, berget stands alone as a regular noun in its definite form ("the mountain").

Why is the past tense of gömma spelled gömde with only one "m"?

Gömma (to hide) is a regular Group 2 verb, so its past tense is formed by adding -de to the stem (gömm-). However, Swedish spelling rules dictate that a double consonant is usually reduced to a single consonant when followed by another consonant. Therefore, gömmde drops an "m" to become gömde.