English has a lot of tenses (most non-native speakers think we have far too many). I often see authors confused about when to use “has,” “had,” “have,” and “will have,” so here’s a short explanation of the simple past, present perfect, past perfect, and future perfect tenses.
We collected the data. Then we analysed it.
This is simple past. The single action in each sentence (collection & analysis) began in the past and ended in the past.
We have decided to change our method.
He has decided to help me analyse the data.
Above, the present perfect is used to describe a decision that was made in the past but will have an effect on the present or the future. In the first sentence, the decision has been made, but the change has not yet taken place. (If you wrote, “We decided to change our method,” people might think that the method had already been changed.) In the second sentence, again, the decision has been made, but the data has not yet been analysed.
Here is an example of past perfect:
We had collected the data, but then we decided to change our method.
The first action (collected) began and ended in the past, and then triggered another action (the decision). Decided is the main verb, which is in the simple past because it is the final action in a sequence that is now complete. Past perfect tense sets up a sequence of events. It makes clear which action(s) began and ended before the action in the main verb takes place.
We will have finished our analysis by next month.
Will have is in the future perfect. It designates an action that will have been completed sometime in the future.
When you have a single event that started and ended in the past, use the simple past.
When you have an event that began in the recent past but is ongoing, use the present perfect.
When you have a sequence of events that started and ended in the past, use the past perfect.
And when you describe an event that begins in the past or present, but has an ending point in the future, use the future perfect.
If you want to learn more about English verb tenses, I recommend the following site, because it has great diagrams and timelines: