Why isn't "I will walk and then I will run" correct?
Because it is not the simple future tense. It is either future continuous (I will be walking…) or present habitual (I walk…, I do be walking…). For I will walk you’d use coisichidh mi (which in some contexts could also mean I walk habitually).
So why isnt i will be walking and then i will be running correct?
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This is a very frustrating way of teaching. We have not been taught the alternative coisichidh mi. How can we know? I get it wrong every time.
What is frustrating? Duolingo doesn’t expect you to know or use coisidh mi… at this stage (though it doesn’t forbid it for the habitual sense if you know it).