Who, Which and That may be omitted with some frequence. So that this happens, there are three conditions:
- They must relate to the object of the Relative Clause:
"Here I have some books you'll have to read." (which/ that you'll have to read)
- The Relative Clause must not be an adjunct:
"The new president we have elected is fatter than the last one." (who/ that we have elected...)
- Can't come after prepositions:
"The speed at which ship can navigate varies according to the direction of the wind." (which can't be omitted)
That's all, folks!
Next post, exercises.