1．你好！

It is an everyday greeting and is used at any time, on any occasion and by a person of any social status. The reply should also be "你好"(nǐ hǎo).

2．你好吗？

It is a daily greeting and is usually used between acquaintances. The answer would often be "我很好"(wǒ hěn hǎo, I'm very well.).

3．很

Although "很"(hěn) generally means "very", it is not used in exactly the same way as in English. It's often used before an adjective. "很" is a very commonly used adverb. Sometimes "很" does not exactly mean the degree is quite high when it's used before an adjective.

4．吗 and 呢

"吗"(ma) and "呢"(ne) are modal particles. Generally speaking, it's used at the end of an interrogative sentence.