When using present or past perfect tenses, there are two parts to the verb, as you know. The "helping" verb, has/have or had, and the main verb.
Look at this example:
Mary had been very nice to me until last week.
If you wanted to add the word always here, to emphasize and enhance understanding, where would you put it? A general rule in English is to put it between the helping and main verbs. Thus:
Mary had always been very nice to me until last week.
In sentences with a helping verb, adverbs are usually more comfortable when placed immediately before the main verb. There are plenty of exceptions to this rule -- adverbs are a highly variable part of speech in English!
Test yourself (answer at the end):
John has gone to the store, but he can buy what you need if you call him and ask.
Where would you put the word already (in the first clause) and where would you put the word probably (in the second clause)?
As you can see from the test sentence, the same general rule is true for modal verbs... the adverb (in bold) is placed immediately before the main verb:
We can definitely go shopping this week.
You shouldn't even think about doing that!
I would absolutely recommend this movie.
Sometimes, the adverb can be placed elsewhere, but generally speaking, even if there are other choices for placement, you won't go wrong if you put it immediately before the main verb (in sentences also containing a modal or helping verb).
Hope this helps. :)
Answer: John has already gone to the store, but he can probably buy what you need if you call him and ask.