The grammar whose productions are
- $\langle\text{stmt}\rangle \to\text{ if id then } \langle\text{stmt}\rangle$
- $\langle\text{stmt}\rangle\to\text{ if id then } \langle\text{stmt}\rangle\text{ else } \langle\text{stmt}\rangle$
- $\langle\text{stmt}\rangle \to\text{ id }:=\text{ id}$
is ambiguous because
(a) the sentence
if a then if b then c:= d
has more than two parse trees
(b) the left most and right most derivations of the sentence
if a then if b then c:= d
give rise to different parse trees
(c) the sentence
if a then if b then c:= d else c:= f
has more than two parse trees
(d) the sentence
if a then if b then c:= d else c:= f
has two parse trees