The very first thing you learn to say in any new language is, without doubt, how to greet people. And there’s a good reason for that – you can’t possibly strike up (=start) a conversation without saying hello first. Well, technically speaking, you can, but we don’t do that in polite society (=educated and well-mannered people).
Hello is a universal greeting – it works at any time of day and in any situation, especially with people you don’t know well and older people.
Hello Mrs Smith. How are you today?
It’s also the word we use when answering the phone.
Hello. – Hi Jenny, it’s Katy. – Oh, hi Katy!
Hello there can be used to address either one person or a group of people.
Hello there! It’s ages since I last saw you. How have you been?
We also use it rather informally when writing to someone whose name we don’t know (or can’t remember!). For example, you want to buy something on Ebay but have questions to ask the seller. Hello (or Hi) there is a perfectly acceptable way of starting your message.
Then, there’s hi, which these days is by far the most popular greeting among native English speakers as it’s friendlier and less formal than hello. It’s used both in conversation in informal writing, i.e. emails and text messages, with family and friends, colleagues and clients, people who do things for us, such as shop assistants and waiters, and so on. The only restriction in its use is the age difference – don’t say hi to anyone who’s much older than you, unless you’re on intimate terms with them as I am with my parents-in-law, for example.