Prejudice for Prepossession. Literally, a prejudice is merely a prejudgment—a decision before evidence—and may be favorable or unfavorable, but it is so much more frequently used in the latter sense than in the former that clarity is better got by the other word for reasonless approval.
Preparedness for Readiness. An awkward and needless word much used in discussion of national armaments, as, "Our preparedness for war."
Preside. "Professor Swackenhauer presided at the piano." "The deviled crab table was presided over by Mrs. Dooley." How would this sound? "The ginger pop stand was under the administration of President Woolwit, and Professor Sooffle presided at the flute."
Pretend for Profess. "I do not pretend to be infallible." Of course not; one does not care to confess oneself a pretender. To pretend is to try to deceive; one may profess quite honestly.
Preventative for Preventive. No such word as preventative.
Previous for Previously. "The man died previous to receipt of the letter."
Prior to for Before. Stilted.
Propose for Purpose, or Intend. "I propose to go to Europe." A mere intention is not a proposal.
Proposition for Proposal. "He made a proposition." In current slang almost anything is a proposition. A difficult enterprise is "a tough proposition," an agile wrestler, "a slippery proposition," and so forth.
Proportions for Dimensions. "A rock of vast proportions." Proportions relate to form; dimensions to magnitude.