English speakers value politeness over almost everything else. You can speak the most perfect English, but if you appear rude, other people won't want to talk to you.
Politeness helps us to deal with other people easily and smoothly. It helps us get on with strangers in a crowded place (like in the underground) and it helps us get what we want (say "Please" and your transactions get easier). Politeness is something we learn as children, and we expect to see it in other people, too.
But one problem is that if English isn't your first language, it's difficult to know what's polite and when to use a polite expression. So here are some quick tips to help!
1. Don't make orders
We rarely use the imperative form. So instead of saying "Do this!" we'd say "Can you do this, please?" or "Would you mind doing this?"
Using modal auxiliaries helps you to make requests:
Can / Could you...
Would you mind (+ ing)...
"Could you help me with this project?"
"Would you mind moving your suitcase?"
2. Ask for permission
If you want to do something that might inconvenience someone else, ask before you do it!
Do you mind if I...
Is it OK if I...
"Do you mind if I turn on the air conditioning?"
"Is it OK if I turn off the photocopier?"
3. Show respect for other people's opinions
You can also seem too direct when you give strong opinions. Supposing the other person doesn't agree? Then it would be difficult for them to share their opinion with you. So English speakers use a range of "softening phrases" to appear less inflexible.
kind of / a bit
"It's kind of hot in here. Can I open the window?"
"It's a bit too late to go out now. Shall we stay in?"
may / might
"It might not be possible to give you a day off next week."
4. Make it easy for the other person to say no
When you are less direct with other people, you give them space to refuse a request or say "no" without losing face. One way to be less direct is to use past forms:
"I was wondering if we could talk about a pay rise." (past continuous)
"Did you have time to look at my report?" (past simple)
"I wanted to ask you a favour." (past simple)
5. Remember and use the "magic words"
The "magic words" are the words that get you what you want. This is the sort of thing we learn from an early age:
Child: "I want an ice-cream."
Parent: "What's the magic word?"
Parent: "Please can I have..."
Child: "Please can I have an ice-cream?"
The magic words for politeness are:
Please - when we want something
Thank you - when we receive something
Sorry - when we inconvenience someone, do something wrong, or can't help someone
Excuse me - when we interrupt someone, or want to ask a stranger a question
For more tips, check out the Cambridge Dictionary page Politeness. If you need politeness for work, check out this post on how to be polite in negotiations and this post on speaking diplomatically at work.