Mid-position adverbs usually go after auxiliary verbs, after be (is, am, are, was, were) and before other verbs.

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.)