Se utilizan cuando una o más oraciones expresan grados de comparación por semejanza o diferencia. As/ as ... as .../ not as ... as .../ as if/ as though.
as - como
He performed the operation exactly as he was told.
El ejecutó la operación exactamente como se le dijo.
Nobody knows that subject as I do.
Nadie conoce ese tema como yo.
as...as - tan...como
not as...as - no tan..como
His car is as fast as mine.
Su auto es tan veloz como el mío.
They are not as good as you think.
Ellos no son tan buenos como tu piensas.
not so...as - no tan...como
His second book was not so good as the first one.
Su segundo libro no fue tan bueno como el primero.
She was not so beautiful as her mother.
Ella no era tan bonita como su madre.
as if / as though - como si
They treat me as if they didn't know me.
Me tratan como si no me conocieran
Bill started to spend money as though he had won the lottery.
Bill comenzó gastar dinero como si hubiera ganado la lotería.
than - que
They started to have more problems than they expected.
Empezaron a tener m ás problemas que lo que esperaban.
Learning English is more difficult than most people think.
Aprender inglés es más dificil que lo que la mayoría cree.