是 (shì) — to be
是 is Mandarin's verb 'to be', but only in a narrow sense: it links two nouns or pronouns. 我是学生。 = 'I am (a) student.' The same form works for 'I am', 'you are', 'she is', 'we are' — verbs in Chinese never conjugate. The trap: 是 is NOT used with adjectives. You don't say 'I am good'; you say 我很好。 = 'I (very) good.' Adjectives stand alone as their own little predicate, usually with 很 softening the gap.
Verbs don't conjugate — ever
是 is 是 for everyone: 我是 (I am), 你是 (you are), 他是 (he is), 她是 (she is), 我们是 (we are). No endings, no agreement, no tense markings. Same with every other verb. Once you know a verb's pinyin and character, you know all its 'forms'.
是 is for nouns, not adjectives
Use 是 to link a noun to another noun: 我是老师。 = I am a teacher. 他是学生。 = He is a student. But for adjectives — 'I am tired', 'you are good', 'the book is new' — drop 是 and put 很 before the adjective: 我很好。 你很好。 书很新。 很 literally means 'very', but here it's just a placeholder. With no 很, an adjective alone usually implies contrast ('I'm fine [but you're not]').
Greetings
你好 (hello — anytime). 早上好 (good morning, lit. 'morning good'). 晚上好 (good evening). 你好吗? = how are you? — literally 'you good QUESTION'. The standard reply: 我很好,谢谢。 = I'm fine, thank you. 再见 (goodbye, lit. 'again see'). 谢谢 (thanks). 不客气 (you're welcome, lit. 'don't be polite').
Introducing yourself
我是 X。 = I am X. 我叫 X。 = I'm called X / my name is X. 叫 is more natural for names; 是 is fine for things like 'I'm a student'. Ask: 你叫什么名字? = what's your name? (literally 'you called what name').
