Learn Russian: Они много работали днём.

To make the past tense, we drop the -ть from the dictionary form работать (to work) and add a past tense ending. Since the subject is Они (they), which is plural, we add the plural ending -ли, giving us работали.

Does работали change if the group of people working is all female?

No. In the past tense, Russian verbs change based on gender (masculine, feminine, neuter) only for singular subjects. For any plural subject like Они (they), the ending is always just -ли, regardless of the gender of the people in the group.

Why is много placed before the verb?

In Russian, adverbs like много (a lot, much) usually come right before the verb they describe. English normally puts "a lot" at the end of the sentence, but Russian prefers the order много работали.

I learned that "day" is день. Why is it spelled днём in this sentence?

To say that something happens "in the afternoon" or "during the day", Russian changes the noun день (day) into днём. This uses a special word ending (called the instrumental case) to turn a noun into a "when" expression.