From a structural poit of view there are 3 types of clauses:
I. Finite clauses - a clause containing a finite verb (drank, could run, have slept, are dreaming, is written etc)
Finite clauses always contain a subject as well as a predicate except in the case of:
- commands (Sit down!, Be serious!, Take it back!)
- subject ellipsis in coordinate clauses (Miriam wrote the letter and gave it to me to send it)
Nearly all independent clauses in discurssive English are finite.
II. Non-finite clauses - a clause containing a non-finite verb (to bring, having broken, torn)
Non-finite clauses always have the ability to do without a subject, although in many kinds of non-finite clauses a subject is optional.
There are several classes of non-finite verbal construction:
1. Infinitive with "to"
-without subject: The most important thing is to shut down the system before leaving.
-with subject: The most important thing is for us to shut down the system before leaving.
-in anticipatory "it" constructions: It is the most important thing (for us) to shut down the system before leaving.
2. Infinitive without "to" (rare)
-without subject: All he could do was help us carry the luggage to the airport.
-with subject: Rather than he do it, I'd prefer to carry the luggage by myself.
3. Present Participle
-without subject: Slamming the door, he warned us that he might come again.
-with subject: Her ex-husband slamming the door, I felt guilty of what we had done.
4. Past Participle (quite rare)
-without subject: Written with passion, the letter was sent to her beloved by a secret delivery boy.
-with subject: The secretary closed the office and turned off the light, the job done.
When the subject of the participial clause is expressed, it is often introduced by "with":
With the tree growing/grown tall, we get more shade.
III. Verbless clauses:- a clause containing no verbal element at all, but nevertheless capable of being analysed in terms of subject, object, complement or adverbial
Although always in need, he has never asked for help from anybody.
Verbless and commonly subjectless clauses take the ellipsis of clause elements one stage further than the non-finite clause.
The ommited finite verb can generaly be assumed to be a form of the verb "to be" and the subject, when ommited, can be treated as recoverable from context:
Whether (he is) right or wrong, he always comes off worst in arguments.
Verbless clauses can also, on occasion, be treated as reductions of non-finite clauses:
(Being) too sad because of the failure, he didn't feel like watching the TV show.
When the subject is already there, it is the verb alone that has to be supplied:
On Purim customers have bought tons of toys, many of them (being) from China.
As with the participial clauses, the subject is often introduced by "with":
With the tree now tall, we get more shade.