The examples show that some people confuse has/have with had because they fail to understand the essence of the past perfect tense. Remember that when two things happened in the past ...

you will need to use a past tense. In German, you can use the perfect tense. This also exists in English to show that you have done something: The perfect tense: How to talk about the past in ...

For example, the tense may change between the methods section and the discussion section. The abstract is usually in the past tense due to it showing what has already been studied. Example: “This ...