What is about a word ‘сказываю’? Is it not a present tense from Сказать?

Don responds: the present tense form сказываю comes from the imperfective verb сказывать, not from the perfective verb сказать. Depending on the context, сказываю is either conversational or old-fashioned. It’s not a word that you find in the formal writing of educated native speakers nowadays.

I’m almost afraid to ask, but why does говорить have two different perfective forms? Are there other verbs like that?

(I can anticipate the answer that сказать was just a different verb originally, I’m just curious about why/how both forms continue to coexist.)

Don responds: I think it’s best to think of it like this: there are two separate concepts, each with its own pair. The first is the verb “to speak” говорить/поговорить. The second is the verb “to say/tell” говорить/сказать. Since they are semantically quite distinct, they easily maintain separate identities, and one may regard the fact that the imperfectives match as essentially a coincidence. I expect most languages have separate words for the concepts, e.g. German sprechen vs sagen, Spanish hablar vs decir, French parler vs dire, Bulgarian говоря vs казвам, Polish mówić vs powiedzieć.

Of course, it really is not a coincidence that the imperfectives match in this case. They are obviously semantically related since they both address spoken communication. Historically the сказ- route probably had punctiliar aktionsart, where as говор- had more of a processual meaning. (I haven’t studied the issue recently, so my terminology is sloppy here.) But essentially those grammatical descriptions boil down to speaking versus saying.

In short, I advise students to memorize the one pair as “to say” and the other pair as “to speak.” That should keep things pretty straight.