Final answer:
The simple past tense is used to say a completed action in the past.
Step-by-step explanation:
The tense used to say a completed action in the past is the simple past tense. This tense is formed by adding the suffix -ed to regular verbs, while irregular verbs have their own specific past tense forms. For example, in the sentence 'Hey, I saw a dog today,' the verb 'saw' is in the simple past tense, indicating that the action of seeing the dog happened in the past and is completed.