Page 576 - Practical English Usage 3ed - Michael Swan, Oxford

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Speak is often used for communication in more serious or formal situations. I'll have to speak to that boy - he's getting very lazy.
They had a row last week, and now they're not speaking to one another. After she had finished reading the letter, nobody spoke.
lectures etc
Talk is often used for the act of giving an informal lecture (a talk); speak is preferred for more formal lectures, sermons etc. Compare:
This is Patrick Allen, who's going to talk to us about gardening.
This is Professor Rosalind Bowen, who is going to speak to us on recent
developments in low-temperature physics.
The Pope spoke to the crowd for seventy minutes about world peace.
languages
Speak is the usual word to refer to knowledge and use of languages, and to the physical ability to speak.
She speaks three languages fluently.
We spoke French so that the children wouldn't understand. His throat operation has left him unable to speak.
other cases
One usually asks to speak to somebody on the phone (ArnE also speak with). Hello. Could I speak to Karen, please?
Talk is used before sense, nonsense and other words with similar meanings. You're talking complete nonsense, as usual. (NOT ¥tJtt're Sf'mking e6tn1'iete
nlm-sense .. .)
speech (1): stress and rhythm
Stress and rhythm are important elements in English pronunciation. If learners pronounce all the syllables in a sentence too regularly. with the same force and at the same speed. they can be quite hard for English speakers to understand. And if learners are not sensitive to English stress and rhythm, they may not perceive unstressed syllables (especially 'weak forms' - see 616) at all, and this may make it difficult for them to follow natural English speech.
stress
Stress is the word for the 'strength' with which syllables are pronounced. In speech. some parts of English words and sentences sound louder than others. For example, the first syllable of CARpet, the second syllable of insPECtion or the last syllable of conFUSE are usually stressed, while the other syllables in these words are not. In the sentence Don't look at HIM - HE didn't do it, the words him and he are stressed in order to emphasise them. Stressed syllables are not only louder; they may also have longer vowels. and they may be pronounced on a higher musical pitch.
word stress
English words with more than one syllable mostly have a fixed stress pattern.
There are not many rules to show which syllable of a word will be stressed: one
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