Frequency adverbs show how often something happens. Examples are: often, never, always, sometimes, generally, usually, seldom, rarely, ever, hardly ever, frequently etc. Frequency adverbs are usually placed:

a)    Before the principal verb
b)    After the verb be
c)    Between the auxiliary and the principal verb
d)    Before used to and have to

Subject + adverb + main verb + object etc.

I often visit my grandparents.
They seldom go out.
I sometimes read thrillers.
You never help me.
She always comes in time.
He usually admits his faults.

Subject + be + adverb + complement etc.

The frequency adverb goes after be and its forms.

She is always happy.
You are never anxious about me.
He is always late in going to office.
I am often ready for work at 7 am.
I am sometimes asked to address public meetings.
We are rarely invited to dinner parties.

Subject + auxiliary verb + adverb + verb + object etc.

If there is an auxiliary verb, the frequency adverb goes after it. If there are two auxiliary verbs, the frequency adverb goes between them.

I have always been interested in the offer.
They have never offered an explanation.
I have often thought of writing a biography.
He has seldom been on time.
We should never waste such opportunities.
He will always regret having done this.
We should always be ready for compromise.
They will hardly ever take such risks.

Notes

Note that when the auxiliaries are to be stressed, they are sometimes put after the adverbs.

I often am worried about my health.
We never have sought such favors.
You never should utter such words.

Used to and have to are always placed after the adverbs:

We sometimes used to stay up late.
He occasionally used to drink.
You seldom have to worry about your finances.