If you are setting a song to an existing tune, it seems perfectly acceptable to just use the A part if the song fits.

If the song has a chorus, there are two possibilities. Either you can use the B music for the chorus or you can simply use the A music throughout. Both are valid choices.

Being unaware of musical structures does limit choices, but is not fatal. If you write a song and find a tune you like and it fits your song, then the fact that you have only used one section of the tune is not necessarily a fault, it is just that the writer was unaware that they had a choice.

However it is very useful to learn about musical form and it does enable you to make decisions with an awareness of the choices you have available.