Basics about how an English sentence is built
An English sentence consists of a subject, a verb, a completer and a modifier. A simple sentence can only have a verb, and a sentence is simple as long as it consists of one verb.
A sentence in which we cannot find a verb or a subject and it cannot stand by itself is called a sentence fragment. When two or more independent sentences are joined without appropriate punctuation or conjunction then it is called a run-on sentence.
A boy took a book from the library.
‘Boy’ is a subject here, ‘took’ is a verb, ‘book’ is a completer (object here), ‘from the library’ - a modifier.
From linguistic point of view, a sentence consists of an argument and predicate; predicate contains a verb and adjuncts.
So, in our sentence: ‘A boy took a book from the library.’
‘Boy’ is an argument, and ‘took a book from the library’ is a predicate.