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.

This and these are used to talk about people and things which are close to the speaker.

Can you put this book on that table? (Here the book is lying in a position close to the speaker but the table isn’t.)

The relative pronoun that is only used in identifying (restrictive) relative clauses.

The boy who stole the bicycle has been caught. OR The boy that stole the bicycle has been caught.

The English demonstrative pronouns are this, that, these and those.
This and that are singular in number. These and those are plural in number.
Study the following sentences.

Fill in the blanks with appropriate relative pronouns.

1. He got married again two months later ………………….. surprised everybody. (which / where / what)

We can combine two or more simple sentences into a complex sentence by using the joining word that. A complex sentence has one main clause and one or more subordinate clauses. The subordinate clause could be a noun clause, an adjective clause or an adverb clause.

We use who when referring to people. We use which or that when referring to animals or things. Note that that can be used in place of whom when referring to people.