In both cases, there is more than one possibility but they have slightly different meanings 1. yesterday my uncle repaired the car, while I ... 2. yesterday my uncle was repairing the car, while I ... 3. while I am eating, my mum is preparing my lunch 4. while I am eating, my mum prepares my lunch 5. while I am eating, my mum will prepare my lunch #4 and #5 are timeless. They can be interpreted as answers to the questions: "What does your mother do while you are eating?" "What would your mother do while you eat?" #1 and #2 are equally good and have the same meaning, although they "feel" different. #1 reports the fixing of the car as an event, whereas #2 reports the fixing of the car as an activity that occurred over a period of time.

You can use the past simple tense of the verb "repair" in the first sentence, and the present continuous tense of the verb "prepare" in the second sentence. The completed sentence would be: Yesterday my uncle repaired the car, while I was doing my homework. While I am eating, my mum is preparing my lunch.