Теория:

Study the information about the adverbs of frequency.
  
Adverbs of frequency tell us how often something happens/is the case, happened/was the case, will happen/will be the case, etc. 
 
There are lots of them. Here are some examples:
always  constantly usually  often sometimes  rarely  never
 habitually normally frequently occasionally infrequently     
 chiefly mostly repeatedly  sporadically seldom  
 predominantly  generally   intermittently    
 typically commonly  spasmodically    
 continuously largely    
  regularly     
 
Where do they come in the sentence?
  
1. If the sentence has one verb in it (e.g. no auxiliary verb) we usually put the adverb in the middle of the sentence, i.e. after the subject and before the verb:  
  
Tom usually goes to work by car.
  
2. The adverb usually comes after the verb "be":  
  
Tom is often late.
  
3. This is not the case if we put the adverb at the beginning or end of the sentence for emphasis. This rule also does not apply to short answers:
 
- Is she usually on time?
-Yes, usually she is.
  
4. If the sentence has more than one verb in it (e.g. auxiliary verb) we usually put the adverb after the first part of the verb:  
  
I can never remember his name.
  
In the sentence with 'have to' we usually put the adverb before the first part of the predicate.
 
We often have to wait for the bus.
  
5. For emphasis we can put the adverb at the beginning or end of the sentence. At the end is unusual - we usually only put it there when we have forgotten to put it in earlier.  
 
Sometimes we go to school by bus.
We go to school by bus- sometimes.
  
Exceptions:
 
1) "Always" can't go at the beginning or end of the sentence.
  
2) "Never", "seldom", "rarely" can't go at the end of a sentence. They only go at the beginning of a sentence in "polemic statements". Then they have to be followed by the word order for questions.
  
6. When using adverbs of frequency in the question form, put the adverb before the main verb.
 
Do you often go to the cinema?
  
Exceptions: "Never", "seldom", "rarely" and other adverbs of frequency with a negative sense are not usually used in the question form.
  
7. When using adverbs of frequency in the negative form, put the adverb before the main verb.
 
They don't often go to the park.
 
Exceptions: "Never", "seldom", "rarely" and other adverbs of frequency with a negative sense are not usually used in the negative form.
Источники:
justaskwelc.blogspot.com
esl.about.com