Ninakupenda is broken up as follows:
NI (first person singular subject marker)
NA (present tense marker)
KU (second person singular object marker)
PENDA (verb meaning "love")
So, ninakupenda means "I love you."
Ninapenda means, "I love." It is usually said "Ninapenda wewe." Wewe is the second person singular pronoun, usually said for emphasis. So "ninapenda wewe" and ninakupenda mean the same thing.