I just came this expression during my daily review:
Vou ajudar com tudo quanto precisar
My inclination would be to use precisa here instead of the infinitive precisar, and I’ve seen a number of similar exercises that use the infinitive of a verb in sentences like this. In this particular case, would precisa also be correct?
@pwsteele, in this case, we are not using the infinitive, but actually the future subjunctive - it just happens to look the same in this case. The sentence is about what the person might need in the future, so there’s a degree of uncertainty there that fits well with the future subjunctive. In any case, the sentence also accepts the present indicative precisa, in which case it already establishes that the person needs something now 
Do these examples have the same explanation?
Vamos à festa quando eu decidir
Vamos pedir a comida quando ele chegar
My inclination is to use decido and chega here, but I can see how there is an implied future action in each case.
Not really. Although the examples also use the present subjunctive, the contextual implications are different.
Example 1:
Vamos à festa quando eu decidir → With the verb in the future subjunctive, we understand you’re talking specifically about this one situation that is expected to happen at some uncertain point in the future. You’ll eventually go to this party whenever you say so. In this context, the first verb vamos works as a future form.
Vamos à festa quando eu decido → With the verb in the simple present tense, it feels like you’re describing a general case or rule. It’s as if you were saying that you generally go to parties when you decide. In this context, the first verb vamos is understood to really be in the simple present.
Example 2:
Vamos pedir a comida quando ele chegar (We’ll order the food when he arrives) → This sentence, with the future subjunctive, is also focused on this one specific future situation.
Vamos pedir a comida quando ele chega → This sentence is actually incoherent. Vamos pedir, as an informal future structure, doesn’t match in terms of timeline with the present indicative chega. If you wanted to form a general sentence here, as discussed for the other example, you’d need to reword it to Pedimos a comida quando ele chega (i.e. all in the present tense).
I think you need to be a linguist to understand some of these concepts. Concepts like future conjunctive and present indicative were never taught when I went to school in Canada. I have a hard time wrapping my head around to these concepts.
It might be useful to add an info item to examples like these in the lessons to explain what’s going on. To me at least it was not clear how these tenses were involved.