Mid-position adverbs usually go after auxiliary verbs, after am / are / is / was / were and before other verbs.
- She has never written to me.
- The discussion was mainly about politics.
When there are two or more auxiliary verbs, the adverb usually goes after the first.
- You have definitely been working hard.
In American English, mid-position adverbs are often put before auxiliary verbs and am / are / is / was / were, even when the verb is not emphasized.
- You certainly have made him angry. (US)
- You have certainly made him angry. (GB)
- You are always late. (GB)
- You always are late. (US)
- America has long been known as a land of opportunities. (GB)
- America long has been known as a land of opportunities. (US)
In British English, mid-position adverbs can go before auxiliary verbs and am / are / is / was / were when we want to emphasize the auxiliary verbs.
- I am really sorry. (No emphasis on am.)
- I really AM sorry. (Emphasis on AM)
In negative sentences, mid-position adverbs generally come before not if they emphasize the negative.
Compare:
- I really don’t like her. (Strong dislike)
- I don’t really like her. (Mild dislike)