hart / heart are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
hart: (noun) United States playwright who collaborated with George S. Kaufman (1904-1961). (noun) United States lyricist who collaborated with Richard Rodgers (1895-1943). (noun) a male deer, especially an adult male red deer.
heart: (noun) the locus of feelings and intuitions. (noun) the hollow muscular organ located behind the sternum and between the lungs; its rhythmic contractions move the blood through the body. (noun) the courage to carry on. (noun) an inclination or tendency of a certain kind. (noun) a plane figure with rounded sides curving inward at the top and intersecting at the bottom; conventionally used on playing cards and valentines. (noun) a firm rather dry variety meat (usually beef or veal). (noun) a playing card in the major suit that has one or more red hearts on it.
hart: hh·AA·r·t
heart: hh·AA·r·t
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)