Modal verbs tell us the speaker's or writer's attitude towards something, such as their opinion about likelihood of something happening.

This whole area is far too complex to deal with here, but here's a general tip:

If you think of modal verbs in pairs - will/would, shall/should, may/might, can/could - the second one in each pair more 'distant' in the speaker's or writer's mind than the first one.

For example, 'I'll give you the money' is a real offer, while 'I'd give you the money' is a hypothetical situation.  'I can tell you' is a direct statement, while 'I could tell you' suggests that there are conditions attached.

'That can be difficult' is more certain than 'That could be difficult'. Likewise, 'Would you help?' is more tentative and more polite-sounding than 'Will you help?'.

You're welcome, Joyce. I've just noticed that there are a few typos and missing words in my answer ('the' likelihood and 'is' more distant). And Happy New Year to you, too.

That's an interesting topic.  The main choices which come to mind are :

a) can / could - for polite requests and possibilities

b) have to / must (in positive statements)

c) will / shall - future simple auxiliary

I can't think of any situations in which you can choose between "will" and "would".  Most of the time, only one modal is correct.  

Happy New Year and really thank you for the comment, Su.Ki. It is very helpful :)

Oops, typo. I would do it.