I am not a teacher, but here are a couple of suggestions... Terms usually reserved for medicine and science have suddenly made their way into everyday conversation. (Otherwise, to me, the sentence does not sound right without more context). Some books arrived in the mail. Otherwise, the sentences sound perfectly natural. Keep going!

Hola Gerardo! #3. You said: “I missed a deadline and lost an opportunity that I don’t think it’s going to happen again.” It should either be: 1. “I missed a deadline and lost an opportunity that I don’t think is going to happen again.” Or: 2. “I missed a deadline and lost an opportunity that I don’t think will happen again.” #7. You said: “I enjoyed the morning sessions of the course, but the afternoons were a bit dull.” I would remove “of the course.” So, it would be: “I enjoyed the morning sessions, but the afternoon ones were a bit dull.” Alternatively, you could say “lessons” or “classes.” Example 1: “I enjoyed the morning lessons, but the afternoon ones were a bit dull.” Example 2: “I enjoyed the morning classes, but the afternoon ones were a bit dull.” #8. “I enjoyed the morning subjects, but the afternoon ones were boring.” You used “subjects” here, but “lessons” might sound a bit more natural in this context also. So, you could say: “I enjoyed the morning lessons, but the afternoon ones were boring.” #9. You said: “The weekday subjects were engaging, but the ones on Saturday were pretty boring.” You said “subjects” again, which could be swapped with either “classes” or “lessons“ which sounds more natural: “The weekday classes were engaging, but the ones on Saturday were pretty boring.” I think your other sentences sound fine—good job Gerardo!