(This is a screenshot of opening of chapter 17 of Shin Nihongo No Kiso. This is done just for the short recap)
1.Shashin o toranaide kudasai. Please don’t take pictures.
Syntax: nai-form verb+de kudasai.
Usage: This is used when you are requesting someone to not do something. Such as the case above, toranai is the nai-form of torimasu which means “to take,to capture”. Toranaide kudasai means “not to take” and the kudasai part add the “please” or a formal request.
2.Mainichi benkyou o shinakereba narimasen. You have to study everyday.
Syntax: verb in na form+nakereba narimasen.
Usage: This is used to express obligation, the need to do something important. Shinai to ikemasen is another form of saying this but it is more formal.
3. Doyoubi no gogo benkyou shinakutemo ii desu. You don’t have to study on Saturday afternoon.
Syntax: verb in na-form+kutemo ii desu.
Usage: This is the complete opposite of nakeraba-narimasen. it is used in stating of actions that is not so important to do.