I Joined an ENGLISH Onlineonline English Course

If "ENGLISH Online" is the name of the course, then keep it like that, but change the "an" to a "the".

I joined an online English online course yesterday.

The adjective order the other user was referring to: https://dictionary.cambridge.org/grammar/british-grammar/adjectives-order

I hope the course will help me to improve my English.

If I get the certificate ofpass this course, I will get a lot of job opportunities.

Alternative with "certificate":
Having the certificate of having finished this course should get me a lot of job opportunities.

I don't know why I'm still havemaking so many grammar mistakes, though,. I've studied so many grammar rules.

Your grammar isn't bad. I added the "so many" because everyone makes grammar mistakes time to time.

I hope after I haveonce I finished the course my English level will be more advanced.

Grammar is perfect here, but it sounds a bit rigid.

Yesterday, I joined an online English online course yesterdaycourse.

I'm not sure how to explain this, but there's a certain pattern as to how certain adjectives should be placed and it helps make sentences flow better.

I hope the course will help me to improve my English.

Alt: I hope this course will

If I finish the course and get thea certificate of this course, I will get a lot of job opportunities.

Alt: If I finish the course and get a certificate, it will open a lot of doors and I will get much more job opportunities.

I don't know why I still have grammar mistakes, though, I'veunderstand why despite having studied so many grammar rules, I still make mistakes.

Since you've already clarified you're referring to grammar, it's not necessary to say "grammar mistakes."

I hope that after I have finished the course, my English level will be more advanced.

I Jjoined an ENGLISH Onlineonline English Course.

Placing "English" before "online" makes the sentence flow better.

I joined an English online course yesterdayYesterday, I joined the course.

As you have already mentioned joining the course, you do not need to repeat it. Instead, just write out when you have.

If I get thea certificate ofrom this course, I will get a lot of job opportunities.

This sentence was mostly correct, however in the first part, some of the words you used did not flow as well as it could have.

I don't know why I still have many grammar mistakes, though,. I've studied so many grammar rules.

The usage of "though" in this case makes the sentence sound weird. It is better to instead make two separate sentences.