你 (nǐ) means “you” (singular). It’s pronounced in the third tone but if you listen to Chinese say, it sometimes sounds like it’s pronounced in the second tone. Traditionally, 你 (nǐ), with the “human” (人) radical, is for males or both males and females and 妳 (nǐ), with the “female” (女) radical, is specifically for females. In simplified Chinese, only 你 (nǐ) is used.
To make 你 (nǐ) plural, simply add the 們 (pronounced mén or men) character to form 你們 (nǐmen). The 們 character has the “human” (人, rén) radical, just like 你 (nǐ), and next to 人 is the “door” (門, mén) character. You can add 們 to form the plural: 我 (wǒ), I, plus 們 is 我們 (wǒmen), meaning we; 他 (tā), he, or 她 (tā), she, plus 們 becomes 他們 (tāmen), all male or some male and some female, or, 她們 (tāmen), all female.
們 is pronounced just like 門. Though not always correct, sometimes you can guess the pronunciation of a Chinese character by following this saying: 有邊讀邊，沒邊讀中間 (Yǒu biān dú biān, méi biān dú zhōngjiān), meaning “read the side if any; read the middle part if there is no side.” This is the case for this character. Maybe you can remember this character that makes words plural this way: A man stands next to the door. The door represents the way to a room full of people.
很 (hěn) means very, quite, highly.
聰明 (cōngmíng) means bright, clever, intelligent, or wise.
If you have a friend who always does well on his exams, you might say to him, 你很聰明 (Nǐ hěn cōngmíng).