Answer:
A complete version of the sentence above could be as follows: After writing all night, Sandra finally had a good introduction, but she knew she could do better.
Explanation:
The complete version of the sentence is a compound-complex sentence. A compound-complex sentence is made up of two independent clauses and one or more dependent clauses. In the sentence above, the two independent clauses are the following ones: "Sandra finally had a good introduction" and "but she knew she could do better". These sentences can stand alone as sentences because they represent a complete thought; they do not depend on any element to complete their meaning (both have a subject and a predicate).
On the contrary, "after writing all night" is a dependent clause. A dependent clause is a group of words that has both a subject and a verb but cannot stand alone as a sentence (see 1 and 2). 1 is a dependent clause, while 2 is not.
1) *After writing all night
2) Sandra finally had a good introduction
