Identify the independent clause: As he moved quickly up the field, Roberto was in a position to score.

Roberto was in a position to score.

Explanation:

Independent clauses are groups of related words with a subject and a verb, that express a complete thought and, therefore, can stand alone as full sentences. The sentence provided consists of two clauses: a dependent one, which is As he moved quickly up the field and an independent clause: Roberto was in a position to score (this latter can stand alone as a full sentence: it expresses a complete thought).

Roberto was in a position to score is the independent clause