"Today I am a boy, but tomorrow I will be a man."

Translation: Hoje sou um menino, mas amanhã serei um homem.

[Jackie-da-China]

what's the difference between"serei"and "sera"??

[Paulenrique]

Serei is the conjugation for "eu" (1st singular person) and será for the 3rd singular person (ele, ela, você, isso, aquilo, etc)

[G.P.Niers]

Why is ‘ser’ used here instead of ‘estar’ even though the sentence makes it clear that this is a temporary kind of ‘to be’?

[Paulenrique]

Although it is temporary...it lasts longer...

[G.P.Niers]

Okay, I guess I'll have to memorise it.

[Davu]

Maybe it will help you to memorise it if you think of these states as being similar to jobs. Jobs aren't necessarily permanent but people use "ser" to describe their occupations. Even a student (definitely not a long term thing) would say "Eu sou estudante".

[G.P.Niers]

Yep, that certainly helps.

[forsty01]

I know that phrasal future tense (ir+infinitive, "eu vou ser") is commonly used in Brazil. Other than knowing that this exercise is practicing normal future tense ("eu serei"), how one know when one MUST use normal future tense rather than phrasal future tense?