7. He (not wash) his hands. They’re very dirty. 8. Mum’s really angry. We (not tidy) our room! 9. I can’t play with my friends this evening. I (not finish) my homework. 10. I (not visit) New York for three years. 11. Where’s Alison? We (not see) her since yesterday. 12. Dad (not take) a holiday since last August.

7. He is not washing his hands. They are very dirty.
Explanation: The sentence is in the present progressive tense, which is used to talk about actions happening at the moment of speaking. Since the hands are dirty in the present, it implies that the person is not currently washing their hands.

8. Mum is really angry. We have not tidied our room!
Explanation: The sentence is in the present perfect tense, which is used to talk about actions that started in the past and continue up to the present. In this case, not tidying the room happened in the past and is still relevant in the present, causing Mum's anger.

9. I can't play with my friends this evening. I have not finished my homework.
Explanation: The sentence is in the present perfect tense, which is used to talk about actions that started in the past and continue up to the present. The person has not finished their homework, indicating an incomplete action that affects their ability to play with friends.

10. I have not visited New York for three years.
Explanation: The sentence is in the present perfect tense, which is used to talk about actions that started in the past and continue up to the present. The speaker has not visited New York for a period of three years.

11. Where's Alison? We have not seen her since yesterday.
Explanation: The sentence is in the present perfect tense, which is used to talk about actions that started in the past and continue up to the present. The speaker has not seen Alison since yesterday, indicating that they have not had any recent sightings of her.

12. Dad has not taken a holiday since last August.
Explanation: The sentence is in the present perfect tense, which is used to talk about actions that started in the past and continue up to the present. Dad has not taken a holiday since last August, which implies that he has not had any vacations in the time that has passed since last August.