Tồi với Tệ có khác nhau không?
Why don't you need to say "phải" between "không" and "tệ"?
the pattern is usually "không" + adjective/verb, but an exception is made for "là" (which becomes "không phải là"). there's no là in this sentence
What's the difference between tệ and xấu?
Tồi = tệ = xấu ... all mean 'bad'