If it's the use, in the present perfect, then there is not a lot to understand. It's simply a form of the verb - and they are used to different forms by now, having encountered, for example, walk, walks, walking.
If it is the form, then that is very straightforward with regular verbs. You need to ensure the correct pronunciation of the -(e)d ending, but I am sure you know that. Irregular verbs, unfortunately, just have to be learnt.
If it's the combination with HAVE to form the present perfect, then they have also encountered combinations of auxiliary + main verb with the negative and interrogative of the present simple (DO) and the present progressve (BE).
Is there anything specific that is causing difficulty?
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