Words are mainly divided into form-class words and function-class words. Nouns, Verbs, Adjectives and Adverbs belong to the form class whereas Articles, Determiners, Auxiliaries and Modals belong to the function class.
Form class words have only one pronunciation, but function class words have many pronunciations. This brings us to strong and weak forms. The function class words are generally used in their weak form in connected speech.
For example: What do you want to do now?
In the sentence, the first do is a weak form, and the second do is strong (stressed).
/wɒt də juː wɒnt tə duː naʊ/
Thus, the use of weak form is an essential part of English speech and one must learn to use the form of function class words to acquire the rhythm of English speech.