A: where are you going now?
B: I am going to the yard to collect your shorts and put them into the washing machine.
A: How many cups of the detergent powder shall I add?
B: 4 parts of all 5 part of the lines illustrated on the cup, I think would make it enough,would not it?
Here are my suggestions:
A: Where are you going? Unless "now" is being emphasized, one could infer that the question is regarding "right now" since it's a direct question to someone else in the act of leaving. If the person had left several times before to different places, then "now" could be added to mean "this time".
B: I'm going to the yard to get your shorts and put them in the washing machine. Conversational English: "I am" => "I'm". "gather" => "get". "put them into" => "put them in"
B: I'm going to get your shorts so you can wash them. In conversational English, a lot of information/context can be inferred based on the entire situation. If the person is going to wash shorts, then it's not really necessary to say they are going to the yard or that they are going to "put them into the washing machine".
A: How much detergent should I use? "How many cups" => "How much"; "of the detergent powder" => "detergent"; "add" => "use"
B: If you filled the cup to the 4th line, don't you think that would be enough? Posed as a rhetorical question, PersonB already thinks that's the right answer. It might (or might not) be slightly impolite depending on situation and tone.
B: Fill the cup to the 4th line. I think that would be enough. Do you? This is a more sincere question, indicating a willingness to entertain the other person's thoughts. Perhaps indicating PersonB is slightly unsure of the answer.