ᐊᑎ ati-
(preverb)
used as a marker of inception or progression (e.g., to begin to, to become, to proceed, etc.)
Examples
ᐁᒀᓂ ᒦᓇ ᐁᑎ ᑕᔑ ᑮᐌᓛᒋᒃ ᑮᐌᑎᓄᐦᒃ, ᒥᒍᓂ ᓈᑲᓈᑫ ᐁ ᐱᒥᓛᒋᒃ᙮
That is when they begin migrating northward again, flying entirely non-stop.
George Quachegan [2012]
ᑫᑲ ᓂᑮ ᐊᑎ ᑳᐦᒋᑎᓀᓐ ᐊᐱᔒᔥ ᐋᐸᑎᓯᐎᓐ᙮
Eventually I started succeeding at obtaining a bit of work.
Gabriel Kiokee - Cree Legends & Narratives [1995]
ᑮ ᐊᑎ ᓵᒥᓀᐤ ᒫᑲ ᒥᓯᐌ ᐁ ᓛᓛᔑᓈᑦ᙮
He proceeded to lay a hand on him, stroking him all over.
Harvey Smallboy - Cree Notes & Texts collected by Truman Michelson [1935]
Etymology
from the lexical root √atim, via the regular process of final consonant deletion