4. Certainty and Uncertainty

To express certainty, we can use the following expressions :
   - I know that ...............
   - I'm certain that .............
   - I'm quite certain  ..............
   - We absolutly sure that ...........
   - It's positive that ...............
   - I have no doubt about ..............
   - You might be right about ..............
   - I really believe that .................
   - etc.
Now, how do you express uncertainty ?
You could use the expressions below to express uncertainty as :
   - I'm not certain/sure/convinced that ...............
   - We have doubt about ...............
   - There's some doubt about ................
Note : 
a. The word " that " is followed by clause.
    for the example        : I'm certain that she can win the game 
                                                                     clause
     Clause is a group of word consisting of subject, predicate
b. The word " about " is followed by a noun.
    for the example        :  - There's some doubt about his ability.
                                     - I have no doubt about Ani's story.

3. Bargaining

In a departement store
Shirley          :  This is nice material. What is it ?
Shopkeeper  :  It's cotton  
Shirley          :  How much is it ?
Shopkeeper  :  It's very good material. It's four dollars a metre 
Shirley          :  Four dollars! That's too expensive. But it's very 
                        attractive.
Shopkeeper  :  How much can you pay, madam ?
Shirley          :  I'll pay three dollars a metre.
Shopkeeper  :  I'll make a special price for you, madam. 
                        Three dollars seventy five a metre.
Shirley          :  How about three dollars fifty ?
Shopkeeper  :  Just for you, madam. Three dollars fifty cens a metre.

2. Expression used for giving compliments to others

Many kinds of the expressions are used to give compliment  to others as below :
   - Wonderful.
   - Congratulation 
   - You look handsome/beautiful tonight
   - Wow, Fantastic.
   - That's great
   - Well done.
   - How beautiful you look.
   - Wow.. beautiful.
   - What a good appearance I see.
   - You look great.
   - Good job.
   - etc
Did you ever get compliment by your friends ? 
What did you say to respond it ? 
If someone gives you compliment you must respond it as :
   - Thank you
   - You've made my day
   - Do you really think so ?
   - It's kind if you say that.
   - Thanks I need that
   - etc

1. Expression giving invitation

Generally all of us have activities that need to attended by others. So you have to invite them to come and join. You might invite them orally (informal) or writing (formal). It's depend on situation where are you at the day. Might be you met your friend when you were jogging, you invite them to come and join for lunch/dinner/birthday party/ etc orally (informal
Here are the examples of giving invitation :
   - Could you come to my party ? 
   - Would you mind coming over on Saturday night ?  
   - Would you like to come to my birthday party  on .......... ? 
   - How about going out for lunch to gather this afternoon  ?
   - I'd like you to ......... ?
   - Can you ............ ?
   - How would you like to ................... ?
   - Would you like to come to my place for dinner tonight, please ?
   - Why don't you join me for lunch/ dinnner ?
   - I'd like to invite you to see our school/factory/office/ while you're in Bali/etc?
   - Would you come with me to the cinema tomorrow ?
   - etc.
If you got problem how to write to invite them, you could visit this link :
How to make birthday invitation design ?
Here are the samples of birthday invitation design :
        
You could visit the link below to get many kind of invitation design :
What is your response  ?
·  Thank you for your invitation I'll happy to come/join/dinner/lunch/etc.
·  That's very kind of you.
·  That sound wonderful, I'll happy to come/ join/ etc.
·  I'd love to go with you.
·  Certainly, I'd love to go/ come/ join/ etc.
·  etc.
But if you refuse/decline an invitation, remember to thank the person for the invitation and give a reason for refuse/decline. You don't  need to give a detailed reason, it's just a general reason. 
Below are responses if you refuse/decline an invitation : 
.  Thank you but I'm sorry, I'm affraid I can't...
·  Thank you for your invitation but I don't think I can make it.
·  I'd love to but I can't. I have other plans.
·  That's very kind of you, but I'm affraid my schedule is booked up all next week/
   nex Sunday/ tomorrow/ Saturday night/ etc.
·  I'm sorry, but I have other plans. Thank you anyway. 
.  I'd like to, but tomorrow I would prefer to stay at home.