The question is so basic that I'm not even sure how to express the nature of my problem. Still, it confounded me to the point I just had to ask it here.
Is there any grammar issue in any of the following sentences? Or are they all considered correct and idiomatic English?
I'm here for just a month.
I'm staying here for a month.
I will stay here for a month.
I am going to stay here for a month.
I will be staying here for a month.
I'm asking mainly because I've found an opinion on another language forum, stating that both 1. and 3. sound awkward - yet I can't understand why.