Jesus is talking to a group of people about what they as individuals should or should not do. In verse 16 all occurences of "you" are plural. In verses 17 and 18 where Jesus teaches them how to behave when they fast, all occurrences of "you" and "your" are singular. In some languages those occurences of "you" may also need to be plural. (See: [[rc://en/ta/man/translate/figs-you]])
The hypocrites would not wash their faces or comb their hair. They did this purposely to draw attention to themselves so that people would see them and give them honor for fasting.
# Truly I say to you
"I tell you the truth." This phrase adds emphasis to what Jesus says next.