What is the opposite of the perfect aspect?

Solution 1:

They’re called non-perfect.

Likewise, the uses of verbs that aren’t finite are non-finite. Honestly, there’s enough terminology in grammar already without inventing an opposite for every term!

Solution 2:

First of all, it is important to point out that the tenses are only three: Past, Present and Future. Tense refers directly to time.

'Simple', 'Continuous' (progressive) and 'Perfect' are aspects, along with the fourth 'perfect continuous' which satisfies the criteria for both 'perfect' and 'continuous'. These refer to state of an action.

As for opposites, different aspects refer to different states. If an action is not continuous, then it is something else. Being different does not make it 'opposite'.

So, put simply: There is no 'opposite' for the perfect aspect, though there are other aspects.

Hope that helps