The tense marker i te is used to say that something was happening in the past:
I te oma au.
I was running.
This indicates continuous action - I must have been running for a duration of time.
Another tense marker indicates that something happened at a particular point in time, perhaps only once. This uses i as the tense marker:
I oma au.
I ran.
The difference between "I ran" and "I was running", or "she sang" and "she was singing" can be quite subtle, but one refers to continuous action and the other refers to a particular moment in time when an event happened.
Kei te kōrero ia.
She is talking.I te kōrero ia.
She was talking.I kōrero ia.
She spoke.Kei te waiata ngā tamariki.
The children are singing.I te waiata ngā tamariki.
She were singing.I waiata ngā tamariki.
The children sang.