Learn

if/then/else — note: NO semicolon before else:

if age >= 18 then
    WriteLn('adult')
else
    WriteLn('minor');

The ; is a separator between statements, so a ; after WriteLn('adult') would end the if entirely and leave else with nothing to attach to — Syntax error, ";" expected or a dangling-else complaint.

Multiple statements need a begin/end block:

if x > 0 then
begin
    WriteLn('positive');
    WriteLn('and non-zero');
end
else
    WriteLn('zero or negative');

Combining comparisons — parenthesise both sides

Logical operators are and, or, not (NOT &&, ||, !). Comparison: =, <> (not equal), <, >, <=, >=.

Pascal gives and/or a higher precedence than the comparisons — the opposite of C. So this looks right and does not compile:

{ if score > 80 and score < 89 then ... }
{ main.pas(5,14) Error: Incompatible types: got "Boolean" expected "Int64" }

The compiler read it as score > (80 and score) < 89. Wrap each comparison:

if (score > 80) and (score < 89) then
    WriteLn('B');

Make this a habit: every comparison joined by and/or gets its own parentheses.

Chaining and case

For ordered bands, chain else if and let each test assume the ones above it already failed:

if score >= 90 then
    WriteLn('A')
else if score >= 80 then
    WriteLn('B')
else
    WriteLn('C or below');

case handles a fixed set of discrete values, and its labels can be lists or ranges:

case day of
    1, 2, 3, 4, 5: WriteLn('weekday');
    6, 7: WriteLn('weekend');
else
    WriteLn('invalid');
end;

There is no truthiness: a condition must be a boolean. if n then is a compile error; write if n <> 0 then.