Sentences without subject and predicate
A 非主谓句 (fēi zhǔwèi jù) does not divide into subject and predicate; it consists of a single word or a short phrase.
Sentences of this kind are especially common in speech.
What makes them distinctive
In these sentences the usual shape:
Subject + predicate
— is not required.
A single word or short phrase already carries the full meaning.
下雨了。
It's raining. / It has started raining.
下雨rain
了marks a new situation
This sentence has no separate subject.
下雪了。
It's snowing. / It has started snowing.
下雪snow
了marks the change of state
上课了。
Class has started.
上课have class
了marks the new state beginning
真漂亮！
So beautiful!
真really
漂亮beautiful
This sentence consists of nothing but an adjectival phrase.
对不起！
Sorry!
This is a set expression; it needs no subject or predicate.
没关系！
It's okay!
It too stands as a sentence of its own in everyday conversation.
The rule in short
非主谓句 = a sentence that does not divide into subject and predicate
Such sentences may express:
a natural phenomenon;
a new situation;
an emotional reaction;
a set spoken expression.
Examples:
下雨了。
It's raining.
真漂亮！
So beautiful!
对不起！
Sorry!
没关系！
It's okay!