I have never watched a play.
The discussion was mainly about money. (NOT The discussion mainly was about money.)
It certainly looks like it is going to rain. (NOT It looks certainly like it is going to rain.)

When there are two or more auxiliary verbs, the adverb usually goes after the first.

He has definitely been working hard.
I have never been to the US.

When adverbs of manner go in mid-position, they are normally put after all auxiliary verbs.

I will have completely finished the work by next June.
I don’t think the repair has been properly done.

In American English, mid-position adverbs often come before auxiliary verbs and am/are/is/was/were.

He probably has arrived now. (US)
He has probably arrived now. (GB)
He ultimately was responsible for the treatment. (US)
He was ultimately responsible for the treatment. (GB)

End-position adverbs

Some verbs are incomplete without adverb complements. For example, after a verb like put, we usually say where something is put. These object complements go in end-position, before other adverbs.

We went to bed early. (NOT We went early to bed.)
They played very well yesterday. (NOT They played yesterday very well.)

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.

Position of Frequency Adverbs

Frequency adverbs can go in the following positions in a sentence:

Before the principal verb

Structure: subject + adverb + verb + object etc.

He never eats vegetables.
I often buy foreign periodicals.
They frequently visit us.
I always read commercial news.
They rarely watch sports channels.
I sometimes watch English films.

After the verb be

Subject + be + adverb + complement etc.

He is always late for office.
We are never invited to dinner parties.
I am always worried about my finances.
He is sometimes consulted by investors.
You are seldom anxious about my health.

Between the auxiliary and the principal verb

When the verb consists of an auxiliary verb, the frequency adverb goes after it.

He has always done justice to all.
I have often thought of starting a business.
I have never forgotten those unfortunate events.
I have sometimes managed to hoodwink others.
We could hardly ever appreciate his conduct.
We shall never seek such favors.
They will always regret having done this.
They will usually take such big risks.

Notes

Some frequency adverbs (e.g. usually, normally, often, frequently, sometimes and occasionally) can also go in the beginning or end of a sentence.

He writes often.
We visit them frequently.
Sometimes I am late for office.

Always, ever, never, seldom and rarely

The adverbs always, ever, never, seldom and rarely need to be placed before the verb. These adverbs cannot be placed in the beginning or end of a sentence.

Adverbs usually go after the auxiliaries. But when the auxiliaries need to be stressed, they are sometimes put after the adverbs.

I sometimes am worried about my health.
We never should do such things.
You always have done justice to all.

Used to and Have to

Used to and have to are always placed after adverbs.

We sometimes used to stay up the whole night.
He occasionally used to write to me.
The fire brigade always has to be ready to face any emergency.

Compare:

Only he lent me five cents. (= He and nobody else lent me five cents.)
He only lent me five cents. (= He only lent me the money, he didn’t do anything else.)
He lent me only five cents. (= He didn’t lend me more than five cents.)
He lent only me five cents. (i.e. to nobody else)

You will have noticed that the meaning of the sentence changes considerably with the change in the position of the adverb only.

Most adverbs, however, can be placed in different positions with no significant change in meaning. There are, nevertheless, some rules regarding the position of adverbs.

Rule 1

When the verb is intransitive (verbs that do not have objects), place the adverb immediately after it.

He walked slowly.
She smiled beautifully.
He spoke fluently.
He arrived late.
They worked hard.

Rule 2

When the verb is transitive with an object following, place the adverb immediately after the object.

She endured the pain bravely.
He offered his help willingly.
She sang the song beautifully.
He drove the car fast.
He did the job well.
He gave his consent immediately.
He took the matter lightly.

Rule 3

Adverbs of Time and Frequency normally come before the verb. Examples are: always, before, often, never, seldom etc.

They seldom visit us.
She never admitted her fault.
(You) always speak the truth.
He is usually late for office.
It is never too late to mend.

Rule 4

Note that when the verb consists of an auxiliary, the adverb goes after it.

They have never invited us to their parties.
I have always wanted to be a writer.
He was greatly praised for his novel idea.
I have not had the time to look into the matter.
We must always obey our parents.
He is still working at the sum.

Rule 5

An adverb which modifies an adjective or another adverb comes before it.

She is very beautiful. (Here the adverb very modifies the adjective beautiful.)
They are highly competitive. (Here the adverb highly modifies the adjective competitive.)
The girl sang so sweetly. (Here the adverb so modifies the adverb sweetly.)

Note that the adverb enough comes after the adjective it modifies.

He was foolish enough to trust her.
She is old enough to do things on her own.
Our army is strong enough to defend our country.

Rule 6

The words only, merely, even, not and never are usually placed before the words they modify.

I merely wanted to know his name.
She was not clever enough to see through his scheme.
He never keeps his word.