Comparison:
1-same characteristics:
Harry is as tall as her mother.
Kelly is as fat as Natalie.
Rule:
S+v+as+adj+as.
2-different characteristics:
a-short adj:
Tom is older than Emily.
Mercedes is quicker than fiat.
Rule:
S+v+adj+er+than.
b-long adj:
Rolls Royce is more expensive than Mercedes.
Maths is more important than geography.
Rule:
S+v+more+adj+than.