Commenting

If you would like to make a comment or insert a remark in an ongoing conversation, it is polite to acknowledge what someone has just said before stating your own ideas.
 
Some phrases

• That's interesting. I think that...
• Interesting point. I would add...
• Hmmm. I hadn't thought of that before.

Questions can also be a useful way of bringing new ideas into a conversation:

• What do you think about . . .
• Have you considered . . .
• What about . . .
• Sometimes a more direct approach is appropriate:
• Can I add something here?
• (Do you) mind if I interject something here? 

source : http://www.eslgold.com