1. I know nothing yet.

2. I don't know about the fact yet.
3. I don't want to read the book yet.
4. I don't cook yet.
5. I haven't cooked yet.

(What about the sentences above? Are they all grammatical? Is 'yet' used only with the present participle?)

They are grammatical, and I can't think of an instance when 'yet' is not used with the present participle.

Thank you.

Then, the following is right?

1. I don't read Harry Potter yet.
(is this correct?)

1. I don't read Harry Potter yet.

(is this correct?) No.

It should be I haven't read Harry Potter yet.

The sentences you provided are grammatically correct. The word "yet" can be used in different ways depending on the context.

1. "I know nothing yet." - This sentence is appropriate if you want to express that you currently have no knowledge, but there is a possibility of gaining knowledge in the future.

2. "I don't know about the fact yet." - Here, "yet" is used to indicate that you currently don't have knowledge about a specific fact, but it implies the potential for acquiring that knowledge in the future.

3. "I don't want to read the book yet." - In this case, "yet" suggests that you do not want to read the book at this point in time, but it leaves open the possibility of wanting to read it in the future.

4. "I don't cook yet." - The word "yet" in this sentence shows that you currently don't have the skill of cooking, but there is a chance that you might learn or start cooking in the future.

5. "I haven't cooked yet." - Here, "yet" is used with the past participle "cooked" to express that you have not cooked at any point in time up until now. It implies that there is a possibility of cooking in the future.

In summary, "yet" is not exclusively used only with the present participle. It can also be used with other verb forms to suggest a current absence or lack of something with the potential for change or development in the future.