An adverb of frequency can go:
before the main verb

He always goes to work by bus.
They never go on holiday in winter.

after the verb be

John is often absent from lessons.
They were usually on time.

after modal verbs, should, can, could, etc.

You should always turn the electricity off.
My boss can never remember my name.

Sometimes and occasionally can go in the front, middle or end position in the sentence.

Sometimes she is late.
She is sometimes late.
She is late sometimes.