Having in mind that "Por" and "Para" can each be translated as “for", please give a suggestion for new learners.
The "easy one" of them is “Para” because it has relatively fewer uses: destinations, recipients, deadlines, and goals. Then, if you can memorize their uses and model sentences, just by discard you will know when to use "Por".
Is the bandwidth (BW) of a sinusoidal signal zero?
Working in the frequency domain, we can give two approaches to this question: 1. Yes, if it is an ideal signal, its Fourier transform is a pulse signal with zero bandwidth. 2. No, if it is a real signal it could have more than one frequency or noise component, so its BW would be different from zero.
If the speed of a body is zero, is its acceleration necessarily zero?
No. For example, the speed of a body thrown upward at its maximum height is zero, but its acceleration is equal to the acceleration of gravity.