2. I'm talking withto my mom and doing my homework now, so I can't help you with your problem.

I think it is more natural to say "talking to" than "talking with" but both are correct and sound fine. It's also normal to shorten sentences by cutting out unnecessary words, so I would say "help with your problem" instead of "help you with your problem" because it just sounds more natural.

3. I've watched a lot of movies and TV series, which related to something about the supernatural.

Usually when talking about genres it is kept pretty short so it would sound weird to say "which related to something supernatural". You could say "supernatural movies/shows" or "about the supernatural" or "about supernatural stuff". I think it also sounds kind of weird to just say "series" on its own but it's definitely correct and I've probably heard it before. I'd usually say "TV series" or "shows" or "TV shows".

5. I told her about his doingwhat he did yesterday.

Your sentence is not really natural-sounding at all. We'd usually say "what he did" or something like that. As in, "I saw what happened yesterday" or "do you like what I wore today?"

7. He'd done the had finished his homework befory the time his mom wentcame into his room.

You should use "come" rather than "go" when the movement is towards the subject. If this sentence implies he was in the room when the mom went in, it makes more sense to use "came" as it centres on him.

Also, "he'd" is 100% correct but I think abbreviating "had" like this sounds kind of weird so I'd probably just stick with "he had".

I think it also makes more sense to use "finished" as this highlights the end of it, like he had done ALL the homework when she came in. I think I'd tend to use "done" for quicker actions, or things considered as quite simple tasks, and "finished" for ones that take more time or when you want to highlight that it had ended specifically.

I would also use "by the time" instead of "before" here. It also kind of emphasises the fact that he managed to finish the homework before she came in.

It sounds quite odd to say "the" homework. It would usually be described by the person who has it, like "his homework", "her homework". I would use "the" when people are talking about homework they share. For example, (to a classmate) "when is the homework due?"

Just to note, your sentence is grammatically correct and sounds fine. These are all notes on what I think sounds natural or not from experience.

8. I'd been watchinged videos about how to cook for several hours until Ibefore I finally understood how to do it.

Your sentence is correct again, but it sounds kind of long and a bit awkward to say. I'd maybe shorten it to just "videos about cooking" instead of "about how to cook" and "knew" instead of "understood".
Like:
"I watched videos about cooking for several hours before I finally knew how to do it."
This makes it sound like a bit less of a mouthful I think, but really these are style choices and not necessary.

9. I'm going to enter thego to a university for programming university.

This sentence is correct, but again sounds kind of weird. How I'd correct it would kind of depend on what you mean though.

If you're just saying you want to study programming at university:
"I'm going to study programming at university" or "I'm going to university for programming" or "I'm going to apply to do programming at university"

If you are going to a university that specifically teaches programming:
"I'm going to go to a university for programming" or "I'm going to go to a programming university" or "I'm going to apply to a programming university"

If you're talking about a specific university it might be:
"I'm going to enter their programming department" or "I'm going to apply to that programming university"

I think in general "enter" sounds a bit weird for university. You can simply say "go to" or "study at" or "apply to". If you're just talking about university in general it makes more sense to say "a" than "the". If you're talking about a specific university though, use "the" or "that".

She'll have tried until she'll pass her exam.

Depends on what you mean here.

Could be:
"She'll try until she passes her exam."
"She'll have tried until she passed her exam."
"She must have kept trying until she could pass the exam."

I will have been cooking for five hours consecutively beforewhen the guests arrived.

You're talking about the future in the first part so the last bit should also be future.

5. I told her about his doingactions yesterday.

This one is tricky because as written it is unclear if the actions happened yesterday or if the telling about them happened yesterday. You can put the "Yesterday" at the beginning of the sentence to make it clear that the telling happened yesterday. The change from "doing" to "actions" is more of a usage issue.

She'll have tried until she'll passed her exam.

I've provided the minimum correction to match the second clause with the use of the future perfect in the first clause, but in real life you are more likely to say "She will try until she passes her exam."

I will have been cooking for five hours consecutively before the guests arrived.

It is sufficient to change "arrived" to "arrive", but in the US you would be more likely to put the time clause first and say "Before the guests arrive I will have been cooking for five hours consecutively."