Hi in Arabic
As in any language, in Arabic there are many different ways to greet someone. A lot more time is spent on greetings in Arabic than is spent in English. I knew some Arabs just learning English and they were amazed at how quickly we greet each other and then move on. They thought it was kind of rude and thoughtless to give such a small amount of time to greetings. To me the length of Arabic greetings seems strange, but I accept the fact that we both have different cultures and place importance on different things.
1. السلام عليكم (assalaamu 'aleykum)
This literally means "peace be upon you". It is probably the most formal greeting in Arabic. The response is و عليكم السلام (wa 'aleykum assalaam). You hear this a lot more often during Ramadan because everyone feels more religious during that month. Also, after saying السلام عليكم you can add و رحمة الله و بركاته (wa rahmat allahi wa barakaatu). This means "and the mercy of God and his blessings", saying may those be upon you as well. This last part was very hard for me to say when I was starting Arabic.
2. اهلاً و سهلاً (ahlan wa sahlan)
This is translated as "hello and welcome". It's less formal than "peace be upon you" and used in less formal situations. You can respond to this greeting with اهلَين (ahlain) which means "two ahlans". However, that response is really only used in the Levantine region (Lebanon, Syria, Jordan, Palestine).
3. مرحباً (marhaban)
This is the closest thing to "hi" that they have in Arabic. With this you can reply with مرحبتين (marhabtain) "two marhabans". People will understand everywhere, but it's only used commonly in the Levant.
4. صباح الخير (sabaah alkhair)
We translate this as "good morning". If you want to take it literally then it means "morning of the good". The response is صباح النور (sabah annour) which means "morning of the light". It doesn't make a lot of sense if you take it literally like that, so it's always just translated as "good morning".
5. مساء الخير (masaa alkhair)
Means "good evening". Like with "good morning", the response is مساء النور (masaa annour), "evening of the light".
6. تحية لك (tahia leka)
This actually means "greetings to you". It's not very common to greet someone with this just as it wouldn't be common to say "greetings to you" in English.