Today is being a productive day. I woke up at 5 a.m and went to the swimming pool. Afterwards I went back to my office and started my work day. I attended some meetings (boring, but ok) and at 9a.m I had my first class in the italki app. Afterwards I made some things on the computer and started to prepare my lunch. After lunch time I had another English class with a tutor community in the italki app, and now I’m here to produce this text to count my activities in this productive day.... The first sentence is not wrong and makes sense - but it sounds more natural in English to say “today has been a productive day so far”. I think “to start” in past tense is difficult to explain (and I’m not a tutor just a native speaker who wants to help). If you say “I started another class” that sounds to me like you started it and something else happened (like the class didn’t finish) or that you are still in the class at the moment. However... you can also say it before you describe an event that has happened (which is why it sounds correct when you say you started your work day. So when you said “I started another italki class” this could have sounded right if there was some description of the class after, like you did with the work day). You can use it in a lot of situations to mean beginning something you are still doing e.g. “I started learning German last year” (implying I’m still learning it). Or something in the past that was interrupted e.g. “I started learning German but I gave up.” I hope that helps (and hopefully another native speaker or someone qualified will explain better than me, I’ve done my best as a non-teacher) - I understood everything you wrote very easily 😁