1. Many artists in Degas' time thought that they could be better writers by copying famous paintings.

2. Many artists in Degas' days thought that they could be better writers by copying famous paintings.

3. Many artists in Degas' times thought that they could be better writers by copying famous paintings.

4. Many artists in Degas' day thought that they could be better writers by copying famous paintings

(Are they all grammatical? Do we have to use 'time' or 'day'? Are they all the same?)

In every sentence, 'writers' needs to be replaced with 'painters.' These sentences don't make sense as they are.

All of the sentences are grammatically correct, but there are slight differences in meaning.

1. "Many artists in Degas' time thought that they could be better writers by copying famous paintings."
Here, "time" refers to the period when Degas was alive or active as an artist. This sentence implies that during the specific era when Degas was creating art, many artists believed that copying famous paintings would improve their writing skills.

2. "Many artists in Degas' days thought that they could be better writers by copying famous paintings."
This sentence is similar to the first one but uses the plural form of "days." It still refers to the specific period when Degas was active as an artist, but it suggests a broader time frame, encompassing multiple days or instances within that era.

3. "Many artists in Degas' times thought that they could be better writers by copying famous paintings."
This sentence uses the plural form of "times," implying that there were multiple distinct eras or phases during Degas' career. It suggests that artists from different periods or stages in Degas' life believed in improving their writing through copying famous paintings.

4. "Many artists in Degas' day thought that they could be better writers by copying famous paintings."
This sentence uses the singular form "day" instead of "times" or "days." It suggests a more specific and singular time period within Degas' career, possibly referring to a particular day or a shorter duration compared to the other sentences.

Overall, all these sentences convey the same general idea that many artists in Degas' time (or days) believed that copying famous paintings would enhance their writing skills. However, by using different forms of time references, the sentences convey slightly different nuances and levels of specificity.