Anodyne, analgesic and anesthetic all denote something used to relieve or prevent pain, all are freely used both substantively and adjectivally, and all have a related noun, anodynia, analgesia, anesthesia, denoting the corresponding state.
Aggravate vs Irritate
Aggravate means to make something worse or more severe.
Afterward vs Afterwards
Afterward is preferred in American usage.
Annul vs Abrogate vs Void vs Vacate vs Quash
Annul, abrogate, void, vacate and quash are used chiefly in legal context and mean to deprive of validity, force, or authority.
All eyes vs All my eye
All my eye—(UK sl.) utter nonsense (also: all my eye and Betty Martin).
All dressed up vs Dressed up
All dressed up—dressed formally; dressed in one’s best clothes.
Afterall vs After all
After all is always two words.
Annoy vs Vex vs Irk vs Bother
Annoy, vex, irk and bother all mean to disturb and nervously upset a person.
Aforementioned vs Aforesaid
Aforementioned and aforesaid, meaning stated previously, should be restricted to legal writing.
Annotate vs Gloss
Annotate, gloss and their corresponding nouns annotation, gloss mean, as verbs, to add or append comment, or as nouns, an added or appended comment intended to be helpful in interpreting a passage or text.
All day and every day vs All the days of one’s life
All day and every day—during the whole of one day and for a number of days afterwards.
Affluent vs Effluent
Affluent refers to the rich.