One common sub-group of the subordinate clause is the relative clause. These are subordinate clauses introduced by a relative pronoun (who(m), whose, which, that). For example

One aspect of relative clauses which sometimes confusion when looking at clause structure is that the relative pronoun that is frequently omitted in a sentence, although it can be reinserted with no adverse effect. The last example given above could be rewritten as

It still contains a main clause and a subordinate (relative, in this case) clause, but the subordinator is omitted. Identifying where the finite verbs are in a sentence often helps to alert you to the presence of a subordinate clause without a subordinator.

In the introduction to clauses, it was noted that although traditional grammars insisted that a clause contained a finite verb, modern grammars include non-finite verb clauses under the general umbrella of clauses. Non-finite clauses, therefore, are those which begin with a non-finite verb form. For example

You might like to try to identify the finite clauses in these sentences too.

Comment clauses are short clauses inserted into a speech or piece of writing which express the speaker's or writer's comment upon the issue under discussion. Most comment clauses are parenthetical; they are asides which could easily be omitted without altering the meaning. They are very common in conversation where a change in tone and modulation often accompanies them. In writing, they play an important role in establishing an effective reader-writer relationship, because they acknowledge the presence of a reader and encourage a reciprocity in the act of reading. Examples of comment clauses include:

in fact
I must say
so to speak
I'm afraid
heaven knows
as you may have heard
you know


Comment clauses are always subordinate, because they could be omitted altogether.