It is difficult to explain, indeed, unless you are a linguist. Yet I'll try. When you are talking about a task or a process that took (or will take) a period of time, supposing that it was or should be accomplished, or about several events that has happened (or will happen) during a period of time, you say "за": -- It will take me a month to finish this work -- Я закончу эту работу за месяц -- It took him a week to have read this book -- Он прочитал эту книгу за неделю. -- He deceived me twice in tree days -- Он обманул меня дважды за три дня. "За" is also used to express that one event has happened or should happen a period of time before another event: -- They must arrive one week before the contest -- они должны прибыть за неделю до соревнования But when you are talking about a state that lasted or will last a period of time, or something that was used or will be used for a period of time, you say "на": -- I will leave for a month -- я уеду на месяц -- The house can be rented for a week or more -- дом можно арендовать на неделю или больше. -- Lend me this book for a week -- одолжи мне эту книгу на неделю. Thus, Одолжи мне эту книгу на неделю. Я за неделю прочитаю ее = Lend me this book for a week. I will have read it in a week.

I am also studying Russian. I might be oversimplifying in my mind, but I picture it as a simple distinction: за + time = in на + time = for "We can do it in five days" (that's how much time we'll need for completion of a task) versus "We can do it for five days" (and then perhaps our supplies will run out so we won't be able to continue). Of course in English, there is another "in", namely: "We can start in five days" (meaning, five days from now), which is через in Russian.

Предлог "за" указывает на время, которое тратиться на совершение указанного действия; время, которое нужно потратить для совершения какого-либо действия. выучила за час -- he aprendido en una hora = me tomó una hora para aprender. Предлог "на" указывает на период времени, в течение которого длится какое-либо событие: я еду на неделю -- voy por una semana = mi viaje durará una semana

даже не знаю как это объяснить

perfect verb -- за час / за неделю / за год // [но за спасибо] (in an hour / in a week / in a year [ but for thanks ]) imperfect verb -- на час / на 2 дня / [но на совесть] (for an hour / for a 2 days / for a year [ but conscientiously ]) ))