Hi! Yes, you can use "so far" in negative sentences. Look at these sentences for example: 1. So far, we haven't had any luck finding a business partner. 2. So far, we haven't heard anything unusual about the new product. 3. So far, I haven't had much trouble learning English without anyone's help. All these sentences are correct. However, from my personal experience, it sounds a little better to use "so far" at the beginning of your sentence like I have in the sentences above.

Thank you, Soheil, again!

Yes, you can use it with Present Simple. For instance: Do you have any questions so far? But about your sentence, it would be better to use the structure "up to this point" instead of "so far". Look at the example: Do you have any questions up to this point? or I don't have any questions up to this point.

And one more thing: you need to use the "present perfect" tense with "so far". It would be ungrammatical to use the simple past. Compare these two: 1. So far, I haven't been able to find a good teacher. (correct) 2. So far, I wasn't able to find a good teacher. (incorrect)