First of all I see many of the Brazilians persisting in using the word "doubt (dúvida)" when an English person would choose to use the word "question.' We do not say "I have a doubt"(tenho uma dúvida) in English when we mean "I have a question..or...I want to ask something." This is important to learn in English. You must say "Question...or ....I have a question."
"If and whether" are similar. Whether differs from if in that the word whether is connected with the idea of contrasting ideas or opinions.
"Whether" is always connected to a negative follow up idea," whereas "if" is not necessarily connected to a negative follow up idea.
In other words English speakers use the word "whether" when contrasting (or comparing) alternatives i.e. positive possibilities against negative possibilities.
When you use the word "whether" you must always use the word "not" in the same phrase.
I am going to the movies whether you like it or not.
I don't care whether you want to me stay home or not.
I make my own decisions whether or not you agree with me.
I hope this helps you a little bit.
Resposta enviada por Bill no Grupo de Estudos do English Experts - https://www.englishexperts.com.br/aberto-o-grupo-de-estudos-do-english-experts/