Whatever (whatsoever) has no inflection. The nominative and the objective are alike and the possessive is supplied by the phrase of whatever (of whatsoever).
The phrase of whichever (of whichsoever) is used instead of whosever exactly as of which is used instead of whose.
The compound relative pronouns may include or imply their own antecedents and hence may have a double construction.
Whoever calls, he must be admitted. [Here he, the antecedent of whoever, is the subject of must be admitted, and whoever is the subject of calls.]
Whoever calls must be admitted. [Here the antecedent he is omitted, being implied in whoever. Whoever has therefore a double construction, being the subject of both calls and must be admitted.]
1. He shall have whatever he wishes.
2. I will do whichever you say.
In such sentences, care should be taken to use whoever and whomever correctly. The nominative (whoever) is required when the relative is the subject of its own clause.
1. He asked whoever came.
2. He told the story to whoever would listen.
3. He asked whomever he knew.
4. He told the story to whomever he met.
The compound relatives are sometimes used without an antecedent expressed or implied.
1. Whoever deserts you, I will remain faithful.
2. Whomever it offends, I will speak the truth.
3. Whatever he attempts, he is sure to fail.
4. Whichever you choose, you will be disappointed.
Note : This construction is closely related to that explained above. “Whoever deserts you, I will remain faithful," is practically equivalent to “Whoever deserts you, let him desert you! I will remain faithful." No antecedent, however, is felt by the speaker, and hence none need be supplied in parsing.
WHICH, WHAT, WHICHEVER and WHATEVER are often used as adjectives.
1. Use what (or whatever) powers you have.
2. Whichever plan you adopt, you have my best wishes.
A noun limited by the adjectives WHAT, WHICHEVER and WHATEVER may have the same double construction that these relatives have when they are used as pronouns. Thus….
1. Take whichever pen is not in use. [Here pen is both the direct object of take and the subject of is.]
WHOSO FOR WHOSOEVER and WHATSO for WHATSOEVER are common in older English.