Learn Spanish (Latin America): Ustedes dejan de mentir y empiezan a quejarse. - You all stop lying and start to complain.

The phrase dejar de followed by an infinitive (like mentir) is a set expression that means "to stop doing something." Without the de, the verb dejar on its own usually means "to leave" or "to let/allow."

The English translation says "lying." Why doesn't the Spanish use the "-ing" form (mintiendo)?

In Spanish, whenever a verb directly follows a preposition (like de or a), it must be in its infinitive form (ending in -ar, -er, or -ir). English says "stop lying," but Spanish uses the infinitive: dejar de mentir (literally "stop of to lie").

Why does empiezan use a instead of de?

Many Spanish verbs require a specific preposition before another action, and these just have to be memorized. While dejar uses de to mean "stop doing," empezar (to start) always uses a before an infinitive.

Why does quejarse end in se if the subject is ustedes?