The -me- form is actually present tense. It's called present perfect, and according to native speakers I interact with, this tense is used when something has happened just now (quoting the explanation they have given me). The focus is actually on the present time. The action is at present finished, but it is not necessarily true that it was finished at a previous moment in time (although that might have been the case, the focus is still on the present). This might seem strange to a westerner, since we tend to conflate the perfective aspect with the past tense. One way of thinking about the -me- form that I find helpful, is to see it, not as describing an action, but a state. It describes a present state resulting from a past action.
But yes, I get the frustration. Duolingo isn't always very good at explaining things before they are presented.
Let's answer myself...
me = Present Perfect marker li = Past Tense marker ta = Future marker na = Present Tense marker
Mtoto ameanguka - the child has fallen
Mtoto alianguka - the child fell
Mtoto ataanguka - the child will fall
Mtoto anaanguka- the child is falling/ falls
To take it a step further:
Present Tense - mtoto haanguki - the child is not falling down
Future - mtoto hataanguka - the child will not fall down
Past Tense - mtoto hakuanguka - the child did not fall down
Perfect - mtoto hajaanguka - the child has not fallen down
Present: watoto wanaanguka -the children are falling down
Future: watoto wataanguka - the children will fall down
Past: watoto walianguka - the children fell down
Perfect: watoto wameanguka- the children have fallen down
Present: watoto hawaanguki - the children don't fall down
Future: watoto hawataanguka - the children won't fall down
Past: watoto hawakuanguka - the children did not fall down
Perfect: watoto hawajaanguka - the children have not fallen down
Disclaimer: I am not a native speaker. This might contain mistakes