The relative pronoun: that/which(things), who/whom(people),whose(usually people). In speaking, that can be used for people but not in formal written English.
restrictive and non restrictive clauses:
Restrictive: weeds that float to the surface should be removed before they decay. (only those that float to the surface)
Nonrestrictive: my car, which is very large, uses too much gasoline.
optional relative clause reduction: removing relative pronoun +verb be + other auxiliary.
Before relative clauses in the passive voice: this is the z value which was obtained from the table areas under the normal curve. Or. This is the z value obtained from the table area under the normal curve.
Before prepositional phrases: the beaker that Is on the counter contains a solution. Or. The beaker on the counter contains a solution.
Before progressive ( continuous) verb structure: the girl who is running down the street might be in trouble. Or. The girl running down the street might be in trouble.
Mr. Jackson, who is a professor, is travelling in the Mideast this year.
Mr. Jackson, a professor, is travelling in the Mideast this year.
other uses of that.
That is optional after –say, tell, think, believe.
John said that he was leaving next week. Or. John said he was leaving next week.
Obligatory that: after the following verbs- mention, declare, report, state.
The mayor declared that on june the first he would announce the results of the search. Mentioned that/ report that/state that.
That clause: it is well known that many residents of the third world country are dying. Or. That many residents of third world countries is dying is well known. That john would do such a thing surprises me.