One fly... (to fly), two files .... (to fly).One girl ...

1) One fly flies, two flies fly.

2) One girl cries, four girls cry.

3) When a wolf sees the moon, it begins to howl.

4) Wolves and sheep are never friends.

5) Our hens lay a lot of eggs.

6) Boys (always) try to catch some balls. 

7) Выбор формы зависит от контекста:

• (Look!) These girls are trying to run away from an angry turkey. Деянье происходит в момент речи, используем the Present Continuous. 
•  These girls (always) try to run away from an angry turkey. Деянье носит повторяющийся нрав (always), используем the Present Simple. 

8) If one goose has one tooth, how many teeth do thirteen geese have? Для рифмовки можно использовать последующий вариант: If one goose has one tooth, how many teeth have thirteen geese.