In the phrase: Vamos à festa quando eu decidir, why is it decidir not decido.
Olá, @kambalange. This is actually the future subjunctive, not the infinitive, although the verb form does look the same in both cases. Here’s another forum topic where your question is answered in detail: Precisar vs. precisa - Portuguese Questions - Practice Portuguese 
By the way, you can always ask questions/report issues from within the units. All you need to do is click on the 3-dot menu to the right of the progress bar and select Report a Problem , before moving on to the next question. It will automatically send us information about that question (including the answer you gave/selected, so you don’t have to copy paste it).