Ir+a+inf. vs future tense
Is it correct to say that the future tense is used when you know when something will take place. Te Llamaré mañana...
And the Ir+a+inf. form is used;
- When you know something will happen, but you don't know exactly when it will. Creo voy a venir manana.
- When you know that something will take place in the near future. Voy a estudiar en la Universidad de Hamburg durante/en el otoño.
I'm so confused as there seems to be a fine line between knowing when to use the two forms...
In my experience, the difference is most native Spanish speakers use the future tense when talking about things in the distant future that may or may not occur. This makes sense when you think about it, because the future tense is also used to express conjecture/probability. Some native speakers have told me that in their countries "Ir+a+inf" is always used in speech, and the future tense sounds very old-fashioned and is used only in writing. In short, I think both pretty much send the same message and you would be understood by any Spanish speaker if you used the future tense, but "ir+a+inf" is much more common. On another note, in your first example "te llamaré mañana" sounds very strange, and I don't think I have ever heard a native speaker from any country say this. In cases where you are promising/telling someone that you will do something in the near future the present tense is normally used. In this case you will generally here "te llamo mañana".
I agree with Pescador about the future tense being used for more distant future actions.
And as shown by his final example you mustn't forget that Spanish often uses the present tense to express the close future. This is one use that we might forget because it doesn't work that way in English, but 'te llamo mañana' I believe would be even more common than 'voy a llamarte mañana' and 'te llamaré mañana' (again agreeing with P) would just sound weird.
Also bear in mind that the future tense is often used in Spanish to express probability in the present or even wonder about something in the present tense:
Estará durmiendo. He is probably sleeping.
¿Quién será esa chica? I wonder who that girl is? (Who could she be?)