The English language has no future tense. To refer to the future, we use various strategies with verbs in present tense (some of them auxiliaries):
I will run
I will be running
I shall run
I’m going to run
I am to run next
I’m running tomorrow
I run next Friday
Because we can conceptualise the future and it plays a big role in our lives, we talk about it often. Naturally, then, the ways we talk about it are subject to pressures of economy, resulting in contraction, e.g.:
I will run → I’ll run
I am going to run → I’m gonna/gon’ run
I’m gonna run → I’mna run → I’ma/Imma run
I’ll is acceptable in Standard English; gonna/gon’ and I’mna/I’ma/Imma are not, though you may see them in dialogue or informal writing or use some of them yourself in everyday speech – gonna is especially widespread.
Recently I came across another form: I’m on [verb]. It seems similar to I’ma and I’m gon’, but I don’t know exactly how or when it developed. Here’s the example I saw, in Elmore Leonard’s novel Mr. Paradise:
‘You know who put the stuff on you?’
‘Somebody close to me, his girlfriend’s punk-ass brother. Is how it goes. But listen, I’m on tell you something, I was scared.’
‘I would be too,’ Delsa said.
I’m on [verb] doesn’t appear to be common, at least in written English, though Google led me to this line from Kathryn Stockett’s The Help: ‘Today I’m on tell you bout a man from outer space.’ And in GloWbE I found: ‘Law have mercy. I reckon I’m on do it.’ (from ‘Entrepreneurs are a first world Phenomenon’ by John Egan).
Based on the few examples I’ve seen, my guess is that I’m on [verb], like I’ma, is originally and still chiefly AAVE. But I’m open to correction, and to other thoughts you might have on it. I’m on wait and see now.
I forgot that Mark Liberman looked at this on Language Log a couple of years ago: ‘Gonna, gone, onna, a — on?‘. He begins with a different example from Elmore Leonard (“I’m on get you to the hospital”, from Raylan), and links to an older post, ‘”on” time’, that deals with the same passage I quote above.
Both posts offer helpful analysis of the construction and its various pronunciations and spellings. Thanks to @f_moncomble for the reminder.
Elmore Leonard’s book Stick (1983) has another spelling:
Barry said, somewhat louder, ‘Well, Cece, it’s up to you. I’m going to ask you once more to leave quietly.’
‘And I’m own ask you to bite this,’ Cecil said…
Toni Morrison’s novel Sula uses various techniques for rendering future tense:
‘When you gone to get married?’
‘I’m a tell you what you need.’
‘One day you gone need it.’
‘But not you. I ain’t never going to need you.’