Поставьте вопросы к предложениям
My friend is doing his lessons now. (общий, альтернативный, Who?)
I saw my friend in the street yesterday. (общий, альтернативный, Where?)

Is my friend doing his lessons now? Is my friend or her friend doing his lessons now? Who is doing his lessons now? Did I see my friend in the street yesterday? Did I or you see my friend in the street yesterday? Where did I see my friend yesterday?