Some words are always uncountable. They cannot be used with the indefinite article (a/an) and they do not have a plural form.
– I need some advice about which course to choose.
– There’s a lot of information availalable on the Internet.
– She’s doing research into language learning.
– There isn’t any news about the elections yet.
Some words may be countable or uncountable, depending on their precise meaning.
– There’s a space between the cupboard and the wall. (countable)
– The astronauts will travel trough space. (uncountable)
Defining relative clauses
Defining relative clauses say which person or thing (or kind of person or thing) we are talking about.
– I’d prefer to go to the session which/that starts at 7.15.
– The matter of which I am speaking now is strictly confidential.
– The man she asked couldn’t tell her anything.
– The apartment where I lived was very cheap.
Non-defining relative clauses
Non-defining relative clauses tell us more about a person or thing that we have already identified.
– On the 3rd of March, which is Saturday, we’re having a party.
– The President, for whom a press conference had been organised, had to leave suddenly.
– The treaty, under the terms of which all prisoners are to be released, was signed yesterday.