Learn Swedish: Munken har bott i klostret sedan han var ett barn. - The monk has lived in the monastery since he was a child.

The main clause uses the present perfect (har bott) because the monk's living situation started in the past and continues today. The sedan subclause uses the simple past (var) because it marks the specific, completed time in the past when that action began.

Is sedan a conjunction or a preposition in this sentence?

It functions as a conjunction here because it introduces a complete subclause with its own subject and verb (han var). If the sentence just said sedan barndomen (since childhood), it would be acting as a preposition.

The base word is kloster. Why is it spelled klostret here and not klosteret?

Ett kloster (a monastery) is a neuter word. When making it definite (the monastery), you add -et. However, words ending in -er, -el, or -en often drop their final vowel before the definite ending to make pronunciation smoother, giving us klostret.

Can sedan be shortened to sen in this context?