Uma frase negativa em inglês com a expressão could

He could not see it.

Tradução: Ele não pôde ver aquilo.

If significa 'se' ussa-se no inicio de frases na maioria das vezes 

1- john is having lunch in the cafeteria now.
is john having luch in the cafeteria now?
john isn't having lunch in the cafeteria now.

2- listen! that's mary who is playing.
listen!   is that mary who is playing?
listen! that isn't mary who is playing.

3- at present they are traveling in south america.
are they traveling in south america  at present?
at present, they aren't traveling in south america.

4-  for the time being, mr. smith is acting as manager of this departament.
is mr. smith acting  as manager of this departament for the time being?
for the time being, mr. smith isn't acting  as manager of this departament.

5-  be careful! the teacher is  watching.
be careful! is the teacher watching?
be careful! the teacher isn't watching.

6-  they are having sales in all the big stores now.
are they having  sales in all the big stores now.
they aren't having  sales in all the big stores now.