Learn Russian: Они быстро одевались.

Reflexive endings change based on the letter right before them. Because the past tense plural ending -ли ends in a vowel (и), we attach the shorter -сь. If the verb ended in a consonant, we would use -ся.

Why is the reflexive ending needed here when English just says 'they dressed'?

In English, 'they dressed' automatically implies they dressed themselves. Russian requires the reflexive ending to show this explicitly. If you just used the non-reflexive verb одевали (without -сь), it would mean they were dressing someone else, like a child.

How do we recognize that одевались is in the past tense?

The letter -л- near the end of the verb is the standard marker for the past tense in Russian. Here, it is followed by -и to match the plural subject Они (they), creating -ли. Then the reflexive -сь is added to the very end.

Could I change the word order and say 'Они одевались быстро'?

Yes, you can! Russian word order is very flexible. Both versions mean 'They dressed quickly,' but putting the adverb быстро (quickly) right before the verb is a very natural and common way to build the sentence.