Some words in English language can be pronounced in two different ways which are called strong forms and weak forms. As an example, the word “that” is pronounced /ðæt/ as strong form and /ðət/ as weak form. There are about forty such words in English language.
It is very possible to use only strong forms in speaking and to be understood. That’s why almost all learners of English don’t even bother learning and using weak forms. However, there are two reasons why someone should learn and use weak forms.
The first reason is that most native speakers of English find an all-strong-form pronunciation very annoying and very foreign-sounding. Such pronunciation makes your English unnatural, something that most learners of English want to avoid.
The second reason is even more important because it definitely affects your ability to understand speakers of English who use weak forms. Since all native speakers of English use weak forms, learners of English need to learn about these weak forms in order to be able to understand what they hear.
Weak forms and contracted forms are not the same thing.
Almost all the words that have both, a strong and a weak form, belong to a category of words that is called function words. These function words do not really have a dictionary meaning as other words have, such as nouns, verbs, adjectives, or adverbs. These function words are usually auxiliary verbs, prepositions, conjunctions, etc.
In certain circumstances they are pronounced in their strong forms but in some situations the weak form is the normal pronunciation.
English language has rules regarding to when the strong form of a word is used and of course when the weak form of a word is used. The rules are fairy simple ones.
Strong form is used in the following situations:
Weak form words whose spelling begins with “h”, such as "her", "him", "have", "has" etc, are pronounced with initial “h” when they occur at the beginning of a sentence. In other contexts the initial “h” is usually omitted.