I finally finished the tenseough excercise I finally finished the tough exercise

"Tense" sounds odd, I would suggest "tough", "difficult" or "stressful" instead, depending on what exactly you mean. If there is a teacher that gave you the exercise, you can say "assignment" instead of "exercise".

I finally finished the verb tense excercise I finally finished the verb tense exercise

It’s best to add context to tell the reader what kind of tense you mean. Otherwise, they may think you’re talking about 紧张 instead of 时态 :-)

Because the later part was more difficult, it took me 3an extra 3 days to finish my goal. Because the later part was more difficult, it took me an extra 3 days to finish my goal.

Because the later part was more difficult, it took me 3 extra days to {finish | reach} my goal. Because the later part was more difficult, it took me 3 extra days to {finish | reach} my goal.

Since I don’t know what your goal was, I’m not sure if “finish” is the right verb. “Reach” sounds better to me.

Because the later part was more difficult, it took me an 3 extra days to finish my goal. Because the later part was more difficult, it took me an 3 extra days to finish my goal.

While the article "an" isn't necessary, it makes the sentence flow better.

Another reason why is that I have been in a period of fatigue, and I often feel tired when studying English. Another reason why is that I have been in a period of fatigue, and I often feel tired when studying English.

Of course, I shouldn't stop my learning plan, but I can slowe down a little to recharge. Of course, I shouldn't stop my learning plan, but I can slow down a little to recharge.

Of couse, I shouldn't stop my learning plan, but I can slowe down a little to recharge. Of couse, I shouldn't stop my learning plan, but I can slow down a little to recharge.

Of course, I shouldn'tdon't want to stop my learning plan, but I canshould slowe down a little to recharge. Of course, I don't want to stop my learning plan, but I should slow down a little to recharge.

It feels like you're trying to convey your intention to continue despite your fatigue with this sentence, and using "don't want to stop" feels better.

For now, I plan to focus on A1(?) vocabulary and modal verbs. For now, I plan to focus on A1(?) vocabulary and modal verbs.

I’m not familiar with the term “A1 vocabulary.”