Usually, the subjunctive is used to express uncertainty, for example: "Talvez eles tenham o que eu quero." "Talvez eu queira ir." (Talvez = maybe) Also, you can use it to wish good things to other people, like: "Sejam bem vindos!" "Tenha um bom dia!" Now, the personal infinitive. I found this good explanation here! Take a look: http://www.learn-portuguese-with-rafa.com/portuguese-personal-infinitive.html Hope I helped :D Good luck, my friend! :)

The personal infinitive You will use it when the form of the Portuguese verb that has no person or number, known as the infinitive, must take a subject (person and number). With expressions like: É necessário... (It's necessary...) É bom... (It's good that) É importante... (It's important that...) É difícil... (It's difficult...) É possível... (It's possible that...) É provável... (It's probable that...) Não é mau... (It's not bad...)