How to use 'say', 'speak' and 'talk'

I often find that my students have difficulties in making sentences with 'say', 'speak' and 'talk'. Well, I hope to be able to enlighten readers on the correct usage of the three words in this post.

When we want to use 'say' in a sentence, we are actually referring to utterance of a person or what comes out of his mouth. You can make sentences such as the following:
a) What did he say to you just now? [What words came out of his mouth that you heard?]
b) I could not hear what he was saying because he did not pronounce the words correctly. [I did not know the contents of what came out of his mouth because the words were not articulated properly]

The word 'speak' refers to vocalising words in a language. You can make the following sentences to bear out this meaning:
a) The small boy can speak five different languages fluently.
b) Speaking is very important if we want to practise pronouncing words in a language that we are learning.

As for the word 'talk', it means converse with someone. These are the two sentences you can make to show this meaning:
a) I have been talking to him for half an hour now.
b) He refused to talk to me yesterday although I met him several times.