"In syntax, verb-second (V2) word order is a specific restriction on the placement of the finite verb within a given sentence. The V2 principle requires that the finite verb (the verb which is inflected for person) appears in second position of a declarative main clause, whereby the first position is occupied by a single major constituent that functions as the clause topic." Or in other words, in order to emphasize "sometimes" you must start the sentence with it, but in doing so, the verb must remain in the second position, pushing the subject back.
I love the way articles like that destroy the explanation in a storm of jargon. Simply put, in the case, when you start the sentence with an indication of time, you have to swap the subject and verb. Fx. "I morgen kører jeg til danmark". The inversion of subject and verb happens when there is some grammatical construct at the beginning that isnt a conjuntion (and also with simple questions fx. "Er du træt?")