Softer and more polite than what?
It's not necessarily doubtful. It could be encouraging. It depends on the tone of voice and the emphasis.
A : I will master English!!
B : YES! You could do it!!
If someone says "You could do it", do you think it implies politeness and it sounds more softer and more polite? I think if I hear that expression, I feel like he is doubtful about my goal. What do you think? Thank you so much as usual.
Softer and more polite than what?
It's not necessarily doubtful. It could be encouraging. It depends on the tone of voice and the emphasis.
I feel it needs "can" unless the other person is giving a condition: You could do it if you applied yourself every day.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
I agree. "You can do it" suggests that you have great faith in the person's ability to do it. "You could do it" sounds as if you are a little more unsure of their chances of success but you are trying to be encouraging anyway.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.
I agree with SoothingDave, it depends on the tone of voice and where the emphasis is placed.