No matter how competent you are, saying any of these phrases in the office are going to give you a bad rep (which can, in turn, ruin you for quite a bit that you're going to need a lot of saving qualities):
1. "I'll try."
You were asked to do something. There's no such thing as "trying"; it's either you do it or you don't. This makes you come off as: 1) half-hearted about the task—and no one wants a half-hearted work and employee; and 2) not confident about your abilities to execute the work. Say you'll do it. Or if the task is terribly huge, offer an alternative.
2. "I can't."
It's normal to think you can't do something that's being asked of you. The work could be daunting (when was it not?) or you got too much on your plate already to take on something else. Instead of saying that you can't, which tells your boss that you're not at all willing to do the task, offer an alternative too. Suggest what you can do.
3. "It's not fair."
This makes you sound entitled, whiney, and naive. You should know by now, as an adult, that life didn't promise to be fair; so why should it be? If you didn't get something you think you deserve from your boss, ask him/her why and what would it take for you to get it or improve.
4. "This is how it's always been done."
This shouldn't be a go-to reason for why you do what you do, or why you refuse to do something you were asked. Saying this line makes you look closed-minded and unwilling to change or improve. Not to mention a blind follower of "tradition," which gives the impression that you don't think about or know why you do what you do, and why you do it in a certain way.
5. "This is just a tiny suggestion..." "I'm not sure if this will do anything..." "This is going to sound stupid..."
If you begin sharing your point in those ways, you'll let your listeners think "Then why should we listen to you? What's the point?" Don't put your ideas down. If you want to sell your suggestion, believe in it first. It's true that in brainstorming sessions ideas need to be fleshed out more, that they can be silly (especially when taken out of the session); but in a brainstorming session, seriously, no idea is stupid.
Also, you don't want to be that person who implies that she herself has stupid thoughts. It just ruins your credibility.
6. "I hate my job."/"I don't like my boss."
You better be ready to say goodbye to your job if you're heard saying this. No one wants a negative person who psyches other people out. You'll be easily replaced by someone who wants the job more than you.
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