Questions & Answers about Это материал, с которым я работал и днём, и ночью.
Why is который in the Instrumental case here (с которым)?
The relative pronoun который connects back to the masculine noun материал, so it takes a masculine singular form. It is in the Instrumental case and paired with с (with) because the verb работать (to work) requires this structure when talking about working with something (работать с + Instrumental).
In English I could say 'the material I worked with'. Why does с go before которым?
In Russian, a preposition that belongs to the relative pronoun must always come directly before it. You cannot leave the preposition at the end of the clause like in English. So 'the material I worked with' strictly becomes 'the material with which I worked' (материал, с которым я работал).
Why are днём and ночью in the Instrumental case without a preposition?
Russian often uses the Instrumental case without a preposition to turn a noun for a time of day or season into an adverb. День (day) becomes днём (during the day), and ночь (night) becomes ночью (at night). Here, they simply describe when the action happened.
Why is there an и before both днём and ночью, and why is there a comma?