É bem simples!
Tudo depende do verbo que você está utilizando
exemplo: I go to school ----> I went to school
exemplo 2: I love my family ----> I loved my family
Os verbos no passado variam em irregular ( como no exemplo 1 em que a palavra muda totalmente) e em regular ( como no exemplo 2 que somente foi adicionado o D na palavra love)
Se for no passado simples flexione o verbo no tempo passado.
No past continuous utilize 'was' ou 'were'.
oi queria ajudar mas nao sei
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.