Question

поставить предложения в вопросит. и отрицат. формы. 1. This dish must be served now. 2. there are some foreign students in the group. 3. that man has been to Australia several times. 4. I have been learning English all my life

Answer 1

1. This dish mustn't be served now. 
Must this dish be served now?

2. There aren't some foreign students in the group. 
Are there some foreign students in the group?

3. That man hasn't been to Australia several times. 
Has that man been to Australia several times?

4. I haven't been learning English all my life
Have I been learning English all my life?

Answer 2

1. This dish must be served now. (Это блюдо должны подать сейчас)
mustn't переводится=нельзя, следовательно мы изменим смысл предложения, если используем этот модальный глагол. Но можно заменить этот модальный глагол его эквивалентом, и тогда мы получим просто отрицательное предложение, не искажая смысла.
This dish (doesn't need to be) doesn't have to be served now. ( Это блюдо не нужно (нет необходимости) подавать сейчас)
Must this dish be served now?
2. There are some foreign students in the group.
Are there ANY foreign students in the group?
There are NO(AREN'T ANY) foreign students in the group.
3. That man has been to Australia several times.
Has that man (ever)been to Australia?
That man hasn't been to Australia.
4. I have been learning English all my life.
I haven't been learning English all my life.
Have you been learning English all your life?