Use the subjunctive after conjunctions of time to describe that something has not yet been observed.

Example of cuando both with and without subjunctive:

Cuando vengan mis papás, los voy a llevar a la playa porque cuando vinieron mis hermanos, fuimos y les gustó mucho.

In English:

When my parents come, I'm going to take them to the beach, because when my brothers came, we went and they liked it a lot.

Use the subjunctive after conjunctions of time to describe that something has not yet been observed.

Example of cuando both with and without subjunctive:

Cuando vengan mis papás, los voy a llevar a la playa porque cuando vinieron mis hermanos, fuimos y les gustó mucho.

Its equivalent in English:

When my parents come, I'm going to take them to the beach, because when my brothers came, we went and they liked it a lot.