Roshan is correct. කතා is from කතාව which is means 'story' or 'narrative'. කරනවා - pronounced k'er'anawa (as in doct'er') means to 'do' thogether it means to 'do story' i.e. speak, tell Say is කියනවා kiyanawa. Many verbs (ක්‍රියාපද) are formed in Sinhala by adding කරනවා to the noun. It usually means do those action. ex. කල්පනා (contemplation) කරනවා = to ponder, contemplate. කරනවා has a conterpart which is වෙනවා which means to become. often they form a pairing. ex. ලෑස්ති වෙනවා = to be ready / ready one self - ලෑස්ති කරනවා - make something else ready, prepare. There are lots of verbs in Literary Sinhala - but many of these are replaced by a noun + වෙනවා / කරනවා in Spoken Sinhala. So as you learn Sinhala, learn the noun + verbs forms first and then the literary forms later. I hope I have helped.

Hi Tony, Although they both mean 'I speak', we don't use 'කරමි' in spoken Sinhala. Eg: Spoken මම සිංහල කතා කරනවා. මට සිංහල හොඳට පුළුවන්. Written: මම සිංහල කතා කරමි.