Is there some trick, pattern, or a given grammatical structure that would tell me when i need to use the spanish word "a" in front of a verb's infinitive? like "voy a ir." why is the "a" needed?

Okay so i'm taking an online spanish course (penn foster), and in the study guide, you have to create a bunch of sentences with words you've learned so far. translating, reading, and those kinds of things are fairly easy, but when it comes to making my own sentences, my brain just kinda waltz's away. anyone got any tips?

Achemist mixes 4 liters of one acid solution with 10 liters of a 40% solution, resulting in a 30% acid solution. what is the value of x, the acid concentration of the first solution

Read and choose the correct verb conjugation to complete the sentence. la señora garcía y usted son científicos. ¿qué ustedes?   estudiamos  estudia  estudiáis  estudian