Both than and as are used in comparisons. Than is used after adjectives and adverbs in the comparative degree. As and that are not used after comparatives.

She is taller than him. (NOT She is taller as me.) (NOT She is taller that me.)

She earns more than I do. (NOT She earns more as I do.) (NOT She earns more that I do.)

As is used to say that two people or things are equal in some way. It is used in the structure as…as and the same…as.

Than and that are not used in comparisons of equality.

The baby’s eyes are the same color as yours. (NOT The baby’s eyes are the same color than yours.) (NOT The baby’s eyes are the same color that yours.)

As and than can introduce clauses in which there is no subject or object pronoun. In this case, they act like relative pronouns.

The meeting was as successful as had been expected. (NOT The meeting was as successful as it had been expected.)

You worry more than is good for you. (NOT You worry more than it is good for you.)

Don’t forget to take your ticket to the airport, as I did last year. (NOT Don’t forget to take your ticket to the airport, as I did it last year.)