• That's OK.
• No problem.
• Don't worry about it.
• No worries.

• Can I ask a favour?
• Can you do me a favour?
• Could you do me a favour?
• Could I ask you to do me a favour?
• I was wondering if I could ask you a favour. (indirect question in the past continuous tense)

http://www.bbc.co.uk/worldservice/learningenglish/radio/specials/142_requests_offers/page9.shtml

• Can I + infinitive...?
• Could I + infinitive...?
• Would I be able to + infinitive...?
• Is it OK/alright if I + present simple...?
• Would it be OK/alright if I + present simple(less formal)/past subjunctive...?

• I hope you don't mind me mentioning this but...
• I wondered if we could have a quick chat about...
• Would you mind (not) + gerund...?
• It would be good if you could...

http://www.bbc.co.uk/worldservice/learningenglish/radio/specials/1113_how_to_news/page5.shtml

http://www.bbc.co.uk/worldservice/learningenglish/radio/specials/1113_how_to_news/page4.shtml

http://www.bbc.co.uk/worldservice/learningenglish/radio/specials/1142_greeting/page2.shtml

• I think (that)...
• My view is (that)...
• In my opinion, ...
• I can assure you (that)...
• The fact is (that)...
• The point is (that)...
• You're missing the point. ...
• Look, ...

• I would like to book an appointment...please.
• I need/want to book an appointment...please.
• Is it possible to book an appointment...please?
• May/Can I book an appointment...please?

• Would you mind + gerund...?
• Are you OK to + infinitive...?
• Is there any chance you could + infinitive...?
• I would appreciate it if you could...
• It would be great if you could...
• If you could..., that would be great.

• Why don't I/you/we/they + infinitive...?
• Why doesn't he/she + infinitive...?
• Shall I/we + infinitive...?
• (Perhaps/Maybe +) subject + should/could + infinitive...
• Let's + infinitive...
• Why not + infinitive...

http://www.bbc.co.uk/worldservice/learningenglish/radio/specials/1113_how_to_news/page3.shtml

http://www.bbc.co.uk/worldservice/learningenglish/radio/specials/1113_how_to_news/page2.shtml

http://www.bbc.co.uk/worldservice/learningenglish/radio/specials/1142_greeting/page6.shtml

Present verb forms can be used to talk about past actions.

http://www.bbc.co.uk/worldservice/learningenglish/radio/specials/1142_greeting/page3.shtml