a / eh 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. 👇
a: (noun) the 1st letter of the Roman alphabet. (noun) the blood group whose red cells carry the A antigen.
eh: NA
a: ah
eh: EH
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)