Learn Spanish (Latin America): Mis amigos patinaron en la calle ayer. - My friends skated in the street yesterday.

Patinaron is the completed past tense (preterite) of the regular verb patinar (to skate). Because the subject is mis amigos (which is "they" or ellos), we drop the -ar ending and add the past tense ending -aron.

Is ayer the reason we use the past tense here?

Yes! Ayer means "yesterday", which is a specific, completed time in the past. This is a big clue that you need to use the preterite tense, like patinaron, rather than a tense used for continuous past habits.

Why is it en la calle? Wouldn't "on the street" use a different word than en?

In English we often say "on the street," but the Spanish word en covers "in," "on," and "at." So en la calle is the standard way to say someone is in or on the street.

I remember learning mi for "my". Why is it mis here?

Possessive adjectives in Spanish have to match the number of the noun they describe. Because amigos (friends) is plural, the word for "my" also has to be plural, becoming mis. If it were just one friend, it would be mi amigo.