Glad to oblige FigTree.
The language is Greek. Not modern Greek but the Greek of those days called Koine Elliniki (See Wikipedia, Koine Greek). Greek verbs, nouns and adjectives change form considerably depending on time and their place in a sentence making the language complicated. The verb forms amartano (Greek: αμαρτάνω) and amartiso (Greek: αμαρτήσω) are correspondingly Present Tense (there is no Continuous and Simple), and Future Simple. If the Future Simple is used, only one act is involved, and that explains 1 John 2:1. If the same act is repeated regularly, the Future Continuous is used unlike English where we stay in the Future Simple. However, the Future Continuous of ancient Greek looks the same as the Present Tense. To illustrate:
(a) I will kill him and go to jail. (One act, Future Simple)
(b) I will kill people whenever I feel like it and I don’t mind going to jail. (Repeated acts, still Future Simple, we don’t say “I will be killing people…”)
In Greek, (b) has to use Future Continuous. And because it doesn’t, we ask forgiveness for one sin.
The Greek Present Tense does not differentiate between Simple (I sin) and Continuous (I am sinning). It is used for a repeated act or for one act in progress now. The context supplies the correct meaning. If used as a Future Continuous it is continuous. Please note that the verb forms I mention are not as they appear in the Greek text but they are the basic forms of the Present and the Future. In English we do that by stating the Infinitive of the verb.
It is interesting to mention that a Greek, reading a modern Greek Bible, does not have a problem with this scripture. He automatically understands it because the translators always translate the two scriptures correctly and the modern Greek language is capable of rendering the nuances.
Sorry for the complicated explanation, but this is Greek. One should be very careful before stating an opinion on complicated scriptures without checking with the original languages because he can easily draw the wrong conclusions.