Using an Array of Boolean

function IsPangram(const sentence : string) : boolean;
var
  seen : Array [0..25] of Boolean;
  i : integer;
  j : integer;
  c : char;
begin
  for i := 0 to 25 do
    seen[i] := false;
  j := 1;
  while j <= Length(sentence) do
  begin
    c := LowerCase(sentence[j]);
    if c in ['a'..'z'] then
      seen[ord(c) - ord('a')] := true;
    Inc(j);
  end;
  result := true;
  for i := 0 to 25 do
    if not seen[i] then
    begin
      result := false;
      break;
    end;
end;

Array [0..25] of Boolean declares a fixed-size array with indices 0 through 25, one slot per letter of the alphabet. Pascal allows any ordinal type as an array index range, so the bounds are written directly as integer literals.

LowerCase converts a single character to lowercase. The single-character overload lives in the System unit, which is always available without a uses clause.

ord(c) - ord('a') maps a lowercase letter to its zero-based index: 'a' → 0, 'b' → 1, …, 'z' → 25. The in ['a'..'z'] guard ensures only letters reach the array, so the index is always in range.

Inc(j) advances the loop variable. Writing j := 1 before the loop and Inc(j) at the bottom of the body is the idiomatic Pascal while loop pattern, equivalent to a for loop with an explicit start and end but allowing the index to be used after the loop exits.

Once the sentence has been scanned, a second loop checks every slot. As soon as an unset entry is found the function assigns false to result and uses break to leave the loop early.