Translation:I am flying to the United States tomorrow.
Actually, I just got this question again, and it really doesn't seem to matter in this case. When using present as immediate future, "I fly tomorrow" and "I'm flying tomorrow" mean the exact same thing. But whether i will ever remember whether it's lecę or latam jutro, who knows?
They're not interchangeable here because the word jutro ("tomorrow") is specified. This is a concrete event, occurring once, tomorrow, rather than flying to the United States whenever, unspecified, in general, or multiple times habitually. The verb here must be lecieć because of the specified event.
It is not a perfect translation but it seems to be working and my brain can cope with the idea, at least until I have had more time to learn and understand this subject.
Imperfective = incomplete = present tense and future tense.
Perfective = completed tasks = past tense.
past tense - I flew yesterday
present tense - I am flying today
future tense - I will be flying tomorrow
Although the infinitive fly can be used in past, present and future tense for now I am concentrating on the past simple, present and future to try to simplify things.