Past simple he (help) his friend now

he (help) his friend now.

здесь есть наречие времени now - сейчас, а потому глагол должен стоять в present continuous - в настоящем длительном времени:

he's helping his friend now. а чтобы поставить сказуемое в прошедшем времени, здесь надо заменить наречие now -сейчас на наречие yesterday - вчера   и тогда получим:   he (helped) his friend yesterday.

Andy washes his car in the garden.

Sherry cleaned the kitchen floor since 6 pm.

Kelly and David are driving to the city centre.

They cutting out some flowers for 10 minutes.

Greg wrote an article since yesterday.

We did our homework since 3 o'clock.

Emma painted the picture for her niece.

You watered this tree for some minutes already!

I wrote a note for my parents.

Polly watched an interview with her favourite actress.

Mason discussed their plans for weekends with his friends.