Comparative is the name for the grammar used when comparing two things.
The two basic ways to compare are using as ... as or than.
She's twice as old as her sister.
I'm almost as good in maths as in science.
Russian is not quite as difficult as Chinese.
It's much colder today than it was yesterday.
This grammar topic is easier than most others.
I find science more difficult than mathematics.
Comparisons with as ... as versus comparisons with than:
When comparing with as ... as, the adjective does not change.
When comparing with than, however, some changes are necessary.