- When the new park opens, I’ll go there every day.
- When the new park has opened, I'll go there every day.
Can we use both of these? What is the difference between them?
The sentences express essentially the same meaning. The first says that upon the event of the park opening, you'll begin to go daily. The second says that under the condition of the park being open, you'll be in the state of going there daily. But these are just more wordy ways of conveying the same message.
Since the present perfect tense ("has opened") isn't required here, the first sentence is a little cleaner. You could also replace "when" with "once" or "after" and "every day" with "daily" without changing the meaning.