Short adverbs of frequency (always, usually, hardly ever etc) can go before the main verb. So can certain other little words such as 'even' 'only' 'just' or 'really', if they modify or relate closely to the verb.
Longer adverbial time phrases (such as 'Once a week') and all other adverbs (such as adverbs of place) generally go at the end, or less commonly, at the beginning of the sentence or phrase.
The one important rule is that you should not put an adverb between a verb and its object.
This is only general guidance, of course. But if you follow these rules, you won't go far wrong.