Question

3-COMPLETE THE SENTENCES USING MUST NOT (MUSTN'T) OR NOT HAVE TO.
a)it is forbidden.you ____________ do that
b)You __________ ask my permission.you do what you want
c)you_________speak to the drive when the bus is moving. it's dangerous.
d)help youself to to anything you want you _________ ask.
e)you __________ park here.there is a double yellow line.
f)pay me back when you can.you _________do it immediately.
g)it's optional.we ________ to go if you don't want to
h)i'ii tell you a secret. you ________ tell anybody else.promise

Answer 1

você tem que ler a frase e entender o contexto no qual ela está inserida. ao compreender a frase, você entenderá se o must deve estar em forma negativa (must not/mustn't) ou afirmativa (must)

por exemplo, na alternativa c, a resposta é must not
you must not speak to the driver when the bus is moving, it's dangerous. 
"você não deveria falar com o motorista enquanto o ônibus está em movimento, é perigoso"

se você ler a frase (ou conseguir compreender em partes) saberá qual a forma correta do must a ser utilizada

espero ter ajudado ;)

Answer 2

a)it is forbidden.you have to do that
b)You don't have to ask my permission.you do what you want
c)you must not speak to the drive when the bus is moving. it's dangerous.
d)help youself to do anything you want you don't have to  ask.
e)you must not park here.there is a double yellow line.
f)pay me back when you can.you don't have to do it immediately.
g)it's optional.we don't have to  go if you don't want to
h)I'll tell you a secret. you must not tell anybody else.promise