According to their structure, simple sentences are divided into:

• one-member and
• two-member sentences.

A one-member sentence is a sentence having only one member which is neither the subject nor the predicate. This does not mean, however, that the other member is missing, for the one member makes the sense complete.

One-member sentences are generally used in descriptions and in emotional speech.

If the main part of a one-member sentence is expressed by a noun, the sentence is called nominal. The noun may be modified by attributes,

Dusk — of a summer night. (Dreiser)
Freedom! Bells ringing out, flowers, kisses, wine. (Heym)
The dull pain and the life slowly dripping out of him. (Heym)

The main part of a one-member sentence is often expressed by an infinitive.

No! To have his friendship, his admiration, but not at that price. (Galsworthy)
To die out there — lonely, wanting them, wanting home! (Galsworthy)

A two-member sentence has two members — a subject and a predicate. If one of them is missing it can be easily understood from the context.

Fleur had established immediate contact with an architect. (Galsworthy)

A two member sentence may be complete or incomplete. It is complete when it has a subject and a predicate.

Young Jolyon could not help smiling. (Galsworthy)

It is incomplete when one of the principal parts or both of them are missing, but can be easily understood from the context. Such sentences are called elliptical and are mostly used in colloquial speech and especially in dialogue.

Best not to see her again. Best to forget all about her. (Abrahams)
What were you doing? Drinking. (Shaw)
Who does it for Mr. George? James, of course. (Galsworthy)
Where were you yesterday? At the cinema.