I believe that a sentence in English must have a subject. (It does not necessarily have an object as for intransitive verbs.) A sentence without a subject is also possible as in an imperative sentence. Here my definition of a sentence is that a sentence should contain one (main) verb.
However, a sentence without a subject and object in English seems not allowed, whereas in some other languages like Chinese, it is indeed possible.
In some cases, if the performer of an action is not clearly defined, e.g. sky, we use it to specify the subject in English. But the subject is often completely waived in some languages as in Chinese. For example, we say "It is raining" (it is for sky), but in Chinese it is said "Rain now (下雨了)" (it is waived). Any thought is welcome.