[Manas Ranjan Mallick]

I am not a professional teacher. In this case,
I think the clarifications given by Mr, Ronbee are very clear. I only like to add the following to strengthen his opinion. This is also to clear the doubts raised by Mr. Sanny. The general as well as universal rule is of course “Adverbs expressing the frequency of an action are usually placed before the main verb but after auxiliary verbs (such as be, have, may, must)”. As we all know there is always some exception to English grammar rules for which, to master English language is considered to be most difficult and challenging task. So here the exception is that certain adverbs of frequency can be placed at the beginning or end of a sentence and 'usually' is one such word.
Normal: Tom usually arrives home late.
Also possible: Usually Tom arrives home late
OR Tom arrives home late usually.
Ofcourse good teachers do not accept the third sentence.