The modal auxiliaries are mostly used with a main verb which always has to be in the infinitive, but note that å is dropped. The auxiliary carries the tense of the sentence. Jeg kan ikke komme i dag. I går kunne jeg ikke gå på jobben fordi jeg var syk. Jeg vil ikke reise til Oslo. Da jeg var barn, ville jeg bare spise godter.
å burde - bør - burde - har burdet, to ought to (do something)
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The past participle is sometimes used. Jeg har aldri kunnet fordra den mannen, (I have never been able to stand that man).
Han har aldri kunnet synge, (he has never been able to sing).
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