A complete verb in a sentence includes all of the words that make up the verb clause. So for example, in the sentence “He was considered strong,” the complete verb would be “was considered.”
The key to differentiating between the complete verb part of a sentence and the rest is to identify the other parts and realize that they aren’t verbs. For example, in the sentence “He was considered strong” the word “strong” is an adjective; it’s not modifying the verb, it’s modifying the subject “he.” It’s possible to figure this out by turning it into a question about the word such as “who was considered strong?” The answer to this question is “he” and not “considered.” That’s how it’s possible to know that the "strong" is an adjective and not part of the complete verb. Complete verb phrases often have similar constructions. For example, in the sentence “I am taken,” the phrase “am taken” is the complete verb. In the phrase “I am going,” the part “am going” is the complete verb. These are examples of the present passive voice. There are many examples of combinations in complete verbs such as phrases like “am taking” for the present progressive.