For instance, "I eat the cake" may not be as precise as "I am eating the cake," or "I have been eating the cake," or "I have eaten the cake."
Notice how each of those sentences has a different meaning.
"I am eating the cake," says that I am eating it right now.
"I have been eating the cake," implies that the process has been going on for awhile, but it might not be going on right at the moment.
"I have eaten the cake," tells a little different story.
Then again, you could say, "I will eat the cake," or "I may eat the cake," or "I must eat the cake," and so on.
Notice that "I eating," just doesn't work! An "-ing" verb when alone (without a helping verb) can not act as the verb that makes a sentence a sentence.
Sing-Along: http://www.youtube.com/watch?v=-F2JWKY63K0
List Of Helping Verbs
am, are, is, was, were, be, being, been
have, has, had
shall, will
do, does, did
may, must, might
can, could, would, should
from this source
Helpful explanation:
http://www.k12reader.com/helping-verbs/
