2) People who are depressed all of the time need to snap out of it.
...Or get more exercise. Or smile. Or read the Bible. Or just not let it get to them. Or... well, you get the idea. Yes, these things can help, whether the depression is clinical or normal. (That's another thing, and let me make this perfectly clear: it is normal to feel depression, even prolonged depression, when things go wrong in your life. Clinical depression is a different entity, a disorder which doesn't necessarily respond to circumstances. Both deserve care and support, but one is an illness.) The trouble is, most people want to solve their friends' problems for them. These solutions and pearls of wisdom are more likely to make a depressed person resentful. Nobody wants their feelings trivialized, by hearing "You just ought to do this, or think this way..." What's more, depressive illness often makes it literally impossible for a person to do such things. If you wouldn't tell someone with a broken leg to run, you can't ask someone to get out more who is terrified of leaving their house.
What to do, in that case? Say, "I love you." Say, "I'll be there for you." And mean it. And do it. Offer a shoulder to cry on or arms to hide in. Be a source of support, not of answers and advice. And you will be helping your friend more than any suggestions ever could.
3) People mostly commit suicide because of failing an exam, because of being dumped by a partner, because of not getting a particular job or not getting into a particular college, etc...
Sometimes this happens... a person commits suicide in response to a single event. But no matter how important this event may be in his life, odds are there's a lot more involved. The number one cause of suicide is untreated depression, period. Depression can be a fatal disease, and suicide is the way it kills. It's harder to see it that way because on the surface, it is the person who does the deed. But it is a person under the influence of a very powerful illness, that makes it impossible for them to see the world clearly.