Some verbs only take ki. For example, the word aroha ("to love") is only used with ki to indicate who is the person being loved:
Kei te aroha te whaea ki tāna tamaiti.
The mother loves her child.
These words that take ki are often called "experience verbs". The following words always take ki:
aroha - to love
āwangawanga - to be worried
eke - to go up
hiahia - to want
kangakanga - to swear at someone
mahara - to remember
mārama - understand
mātakitaki - to watch
mātou - to know
mihi - to greet
mōhio - to know
pā - to strike
pai - to like
pātai - to ask
pīrangi - to desire
tae - to arrive
tatari - to wait
tūmanako - to hope for
tūtaki - to meet
whakaaro - to think
whakahoki - to return
Often a literal translation into English using "to" makes no sense. For example, we would not say "I am worried to the speaking". However, in te reo Māori, the thing that we are worried about (or the thing we wish for, or love, or think about, or desire) is marked by ki:
Kei te āwangawanga ahau ki te kōrero.
I'm worried about speaking.