Use the future subjunctive tense to talk about things that are likely to happen, but that have not yet occurred.
Me liga quando você chegar. (Call me when you arrive.)
Se eu ver ela te aviso. (If I see her I'll let you know.)
Assim que comprar um carro vou lá. (As soon as I buy a car I'll go there.)
Words that almost always trigger this tense:
quando + verb (when you…)
se + verb (if you…)
assim que (as soon as…)