The study regulations ("Studienordnung") will tell you. Sometimes you can choose, then it is up to you. The Online Course Organizer ("Studienplaner") may help you. Keep in mind that not all courses are offered at every term.

Even the English programs include German-language modules. If you think you can handle it, feel free to take it. However there is always an English-only path of modules available for you to take.

Hint: If you don't have a certificate of German language proficiency level A2 at the time of enrollment, you have to get one to conclude your program. It is always a good idea to learn German as early as possible.

Not all courses use OPAL. The course's website or the lecturer will tell you whether a registration is needed and (if so) how to register.

It depends on the specific module. Usually you have to pass an exam. Sometimes, you need a course achievement ("Studienleistung") only. Usually, the lecturer will tell you in the initial session of the course what applies to you, but you will also find the information in the syllabus.

Basically, you can retry a course achievement as often as you need to pass, but you have at most three atempts for exams.

Great. You have to register for this very exam with the examination office ("Prüfungsamt"). The registration period is usually in the middle of term. For most exams, you can register online ("SB Service")

The examination office ("Prüfungsamt") will publish a list, you can find it online

Please, don't bother your professor or instructor by asking.

Quite a few exams have preconditions. E.g., it can be required that you solve a number of assignments, succeed in a lab, or pass another exam, before you are allowed to take part in that exam you would like to register.

The professor gets a list from the examination office ("Prüfungsamt"), where all accepted students are listed. The professor is ordered not to examine any student who is not on this list.

No problem, as long it is your first attempt and you changed your mind early enough: Until one week before the exam's date, you can withdraw and de-register.
But be aware that the de-registration is a matter of the examination office ("Prüfungsamt"); it is not sufficient (but nice) to inform your professor.

In this case you have to get an offical certificate by a medical doctor that states that you are not able to take part in the exam at the very date. You have to hand in the certificate at latest till the third day after the exam to the examination office ("Prüfungsamt")

If it was a written exam, you have the right to check your paper. Usually, there is an official date for post-exams review. But even if you miss it, you can inspect your paper later.

If you find that the corrector did a mistake in your paper's evaluation, point it out. However, be precise and explain exacly, why your result is correct. Don't expect additional points for statements such as "I feel I deserve more points." In addition, for the paper evaluation counts only what you have written, not whether you have had a bad day, you urgently need to pass that exam, or you've thought the exam is on a different topic.

That might be possible in few rare cases. If you really believe so, you can file an official protest to the examination board ("Prüfungsausschuss"). Then, the board will decide in this case.

The examination office ("Prüfungsamt") is the executive examination authority of the University. Its task is to enforce the formal rules. E.g., it has to decide whether you may take part in an exam, or whether you fail an exam for formal reasons.

The examination board ("Prüfungsausschuss") in turn is an elected commission of the faculty, that cares about technical aspects of examinations. E.g., it can grant a request to substitute an exam for another, or decide whether an exam's conditions were valid.

The examination office publish the list. You can show up at the office, or you can check it online.

No, this is not possible due to the official regulations.

If you are registered for an exam and you do not appear, it counts as a fail. The same is true if you miss a deadline for a re-take exam.

Don't panic. You will have two additional chances to take the exam. However, you have to take your second try within one year.

Shit happens. But you have one (but only one) additional chance. But you have to take the exam at the next possible opportunity. I.e., if there is a regular re-take exam you must take it, you can not wait a year again.
If you fail to take the next possible exam, your studies are over!

Well, this is once too often. You have to leave the program.
Try to avoid this situation at almost any cost. Get counceling before it happens. (E.g., contact your professor or your student representatives.)

You can get the topic from any professor of the Faculty of Computer Science. Most professors have topics posted on their websites, but usually also other topics are available, and the professors are open to your own suggestions.

Not directly. You always need a supervisor from the Faculty of Computer Science. However, a joined master thesis is possible where you have two supervisors from different faculties.

Within four weeks after the official assignment you may turn down the topic. However, you can do so only once.

Well, I don't know. However, regarding study regulations, the master thesis is to prove that you are be able to

1. ...solve a state-of-the-art problem...
2. ...that is in the domain of your study program...
3. ...with scientific means...
4. ...within a limited time.

Maybe your topic fails to meet one of theese points.

On a well justified request, the examination board ("Prüfungsausschuss") may give you an extension in special cases. The extension can't exceed six weeks.

Not good. In this case you have failed your master thesis.

However, you can (only once!) retry within one year and with a new topic.

Well, probably because nobody has ask that very question yet. Send a mail to studienorganisation@informatik.tu-chemnitz.de with your question.