Learn Spanish (Spain): Mi hermana necesita dejar de gastar dinero en caprichos innecesarios. - My sister needs to stop spending money on unnecessary treats.

The phrase dejar de followed by an infinitive verb (like gastar) is a set structure that means to stop doing something or to quit doing something. You always need the preposition de to connect dejar to the action you are stopping. Without de, the verb dejar normally just means to leave or to let.

There are three verbs in a row here (necesita dejar de gastar). How do I know which ones to conjugate?

When you have a chain of verbs working together like this, only the first verb is conjugated to match the person doing the action. Here, necesita matches Mi hermana (she). The verbs that follow, dejar and gastar, stay in their basic, unconjugated infinitive form.

In English we say spend money ON treats. Why does the Spanish use en?

Prepositions rarely translate perfectly word-for-word between English and Spanish. While English uses on, the Spanish verb gastar (to spend) pairs with the preposition en. When talking about what you spend money or time on, you will always say gastar en.

What exactly does the word caprichos mean here?

Caprichos translates most directly to whims or cravings. In everyday Spanish, buying a capricho refers to buying yourself a little treat, an impulse purchase, or a small luxury—something you want just because, rather than something you actually need.