Learn Polish: Moja siostra i ja szłyśmy do biura, a oni jechali na wycieczkę. - My sister and I were walking to the office, and they were going on a trip.

szłyśmy has the non-virile ending -łyśmy, which is used for groups without any men. Because the subject is moja siostra i ja (my sister and I), this ending tells us that the speaker (ja) is also female. If the speaker was male, they would use the virile form szliśmy.

Why does the second part use jechali for "they were going"?

The pronoun oni means "they" for a group that includes at least one male (the virile group). Because of this, the verb must take the matching virile past tense ending -li, making it jechali. If it were a group of only females, children, or objects, the pronoun would be one and the verb would be jechały.

Why does this sentence use two different verbs (szłyśmy and jechali) for "going"?

Polish is very specific about how you travel. szłyśmy comes from the verb iść, which specifically means going on foot (walking). jechali comes from the verb jechać, which means going by some kind of vehicle, like a car, bus, or train.

The English translation says "and they were going", but the Polish word is a. Shouldn't "and" be i?