Learn Swedish: Tjuven försvann inte i fjol. - The thief did not disappear last year.

I fjol is a very common fixed expression that also means last year. The word fjol is an old word for the previous year and is almost only ever used in this specific phrase with the preposition i. You will hear both i fjol and förra året used interchangeably.

Can I move the time expression i fjol to the start of the sentence?

Yes, you can say I fjol försvann tjuven inte. Remember the V2 rule: the verb försvann must stay in the second position. Because the time expression i fjol takes up the first spot, the subject tjuven has to move to the third spot, directly after the verb.

Försvann looks irregular. What is its infinitive form?

The infinitive is att försvinna (to disappear). It is a strong verb, which means it changes its core vowel in the past tense instead of adding a regular past tense ending. The pattern is försvinner in the present tense and försvann in the past tense.

Why does tjuven end with -en?

Tjuven is the definite form of en tjuv (a thief). Because it is an en-word, we add -en to the end to change the meaning from a thief to the thief.