The -yam suffix indicates continuous aspect, that is that the action was ongoing. In combination with the present tense, this indicates a true present tense, not just past today.
Examples have been seen in all 3 tenses, but it is less common in the future tense.
pugugumyam
"I am sitting down"
The -i suffix indicates durative aspect, that is that the action took a while. It expects a proposition to follow.
Examples have been seen in the present and past tense. It is never seen following a vowel, yagai does not occur.
pugugumi
"I was sitting..."
It is very common with yab. Yab yabi