4. Correct the mistakes in the sentences. 1 I study now because I've got an exam tomorrow. 2 Are you knowing Katy? She's my sister. 3 My brother watching TV with his friends. 4 My mum is a doctor. She is usually working at weekends.

Here are the corrected sentences: 1. I am studying now because I've got an exam tomorrow. 2. Do you know Katy? She's my sister. 3. My brother is watching TV with his friends. 4. My mum is a doctor. She usually works at weekends. **Explanation**: 1. Using the present continuous "am studying" is more appropriate here to describe what is happening at this time, relative to the exam tomorrow. 2. "Know" is a stative verb (describes a state, not an action), and is usually used in the present simple form. Thus, the correct sentence is: "Do you know Katy?". 3. The present continuous needs the auxiliary verb "is" with "watching", resulting in "is watching". 4. Using "usually works" conveys a habitual action that occurs at weekends rather than a continuous one.