Расскройте скобки: My grandfather (just/retire) and he (enjoy) his free time.

1). My grandfather has just retired and he is enjoying his free time.

Present Perfect. The tense marker just shows that the action happened in the past but has a connection to the present (he feels new in his new status). Present Continuous shows the present state of the old man (he is happy now).

2). Now, he works in his garden and he does all the things he couldnt do when he worked.

Present Simple to show that now it is his regular activity to work in the garden. Past Simple is used to speak about his previous life.